Amicus Curiae Brief — William Trevor Case, Petitioner v. Montana

NO. 24-624

IN THE

Supreme Court of the United States

WILLIAM TREVOR CASE,

Petitioner,

v.

STATE OF MONTANA,

Respondent.

On Writ of Certiorari

to the Supreme Court of Montana

Brief of National Association of Criminal

Defense Lawyers, American Civil Liberties

Union, and American Civil Liberties Union of

Montana as Amici Curiae in Support

of Petitioner

CECILLIA D. WANG

YASMIN CADER

BRANDON BUSKEY

AMERICAN CIVIL LIBERTIES

UNION FOUNDATION

125 Broad Street, 18th Floor

New York, NY 10004

DANIEL B. LEVIN

Counsel of Record

COLIN A. DEVINE

QIAN ZHE (DANNY) ZHANG

MUNGER, TOLLES & OLSON LLP

350 S. Grand Avenue, 50th Floor

Los Angeles, CA 90071

daniel.levin@mto.com

(213) 683-9100

ALEX RATE

AMERICAN CIVIL LIBERTIES

UNION FOUNDATION OF MON- JEFFREY L. FISHER

Co-Chair, AMICUS COMMITTEE

TANA

NATIONAL ASSOCIATION OF

P.O. Box 1968

CRIMINAL DEFENSE LAWYERS

Missoula, MT 59806

1660 L Street, NW, 12th Floor

Washington, DC 20036

i

TABLE OF CONTENTS

Page

INTEREST OF AMICI CURIAE ................................. 1

INTRODUCTION AND SUMMARY OF

ARGUMENT ..................................................... 2

ARGUMENT ................................................................ 4

I.

Protection of Individuals’ Privacy

Interests in Their Homes Is the

Paramount Concern of the Fourth

Amendment. ...................................................... 4

II.

Allowing Officers to Enter Homes to

Render Emergency Aid On Less Than

Probable

Cause

Would

Create

Opportunities for Abuse and Significant

Safety Risks for Officers and Occupants. ......... 9

III.

A.

Opportunities For Abuse........................ 9

B.

Significant Safety Risks ....................... 14

Requiring Probable Cause of Imminent

Injury Will Not Hinder Law Enforcement

From Rendering Necessary Aid in

Emergency Situations. .................................... 19

A.

Probable Cause Is an Objective,

Administrable Standard Familiar

to Officers in the Field. ........................ 19

B.

A Probable Cause Standard Will

Not Prevent Officers or Other

First Responders From Entering

the Home to Address Emergencies.

............................................................... 21

CONCLUSION .......................................................... 25

ii

TABLE OF AUTHORITIES

Page(s)

FEDERAL CASES

Arizona v. Gant,

556 U.S. 332 (2009) ................................................ 2

Brigham City v. Stuart,

547 U.S. 398 (2006) ........................................ 3, 6, 7

Brinegar v. United States,

338 U.S. 160 (1949) ............................................ 7, 9

Buchanan v. Maine,

469 F.3d 158 (1st Cir. 2006) ................................. 18

Buckley v. Fitzsimmons,

509 U.S. 259 (1993) .............................................. 18

Byrd v. United States,

584 U.S. 395 (2018) ............................................ 1, 2

Camara v. Mun. Ct. of the City and

Cnty. of San Francisco,

387 U.S. 523 (1967) .......................................... 5, 20

Caniglia v. Strom,

593 U.S. 194 (2021) ........................................ 23, 24

Cantrell v. City of Murphy,

666 F.3d 911 (5th Cir. 2012) ................................ 21

Carpenter v. United States,

585 U.S. 296 (2018) ................................................ 1

Collins v. Virginia,

584 U.S. 586 (2018) ................................................ 4

iii

Coolidge v. New Hampshire,

403 U.S. 443 (1971) .......................................... 4, 10

Dist. of Columbia v. Heller,

554 U.S. 570 (2008) .............................................. 15

Est. of Chamberlain v. City of White

Plains,

960 F.3d 100 (2d Cir. 2020) .................................. 16

Ferguson v. City of Charleston,

532 U.S. 67 (2001) .................................................. 8

Florence v. Bd. of Chosen Freeholders,

566 U.S. 318 (2012) ................................................ 9

Florida v. Harris,

568 U.S. 237 (2013) .............................................. 19

Florida v. Jardines,

569 U.S. 1 (2013) ................................................ 2, 4

Gaetjens v. City of Loves Park,

4 F.4th 487 (7th Cir. 2021), cert.

denied, 142 S. Ct. 1765 (2022) ....................... 23, 24

Georgia v. Randolph,

547 U.S. 103 (2006) .............................................. 22

Hudson v. Michigan,

547 U.S. 586 (2006) .............................................. 15

Illinois v. Gates,

462 U.S. 213 (1983) .................................... 3, 19, 20

Illinois v. Wardlow,

528 U.S. 119 (2000) ................................................ 2

iv

Kentucky v. King,

563 U.S. 452 (2011) ............................................ 5, 6

Ker v. California,

374 U.S. 23 (1963) .................................................. 6

Kerman v. City of New York,

261 F.3d 229 (2d Cir. 2001) .................................. 20

Kyllo v. United States,

533 U.S. 27 (2001) .................................................. 2

Lange v. California,

594 U.S. 295 (2021) ................................................ 4

Luethje v. Kyle,

131 F.4th 1179 (10th Cir. 2025) ........................... 17

Maryland v. Buie,

494 U.S. 325 (1990) .............................................. 10

Maryland v. King,

569 U.S. 435 (2013) .............................................. 15

Maryland v. Pringle,

540 U.S. 366 (2003) .............................................. 19

McInerney v. King,

791 F.3d 1224 (10th Cir. 2015) ............................ 13

Michigan Dep’t of State Police v. Sitz,

496 U.S. 444 (1990) ................................................ 8

Michigan v. Clifford,

464 U.S. 287 (1984) .............................................. 21

v

Michigan v. Fisher,

558 U.S. 45 (2009) ........................................ 6, 7, 10

Michigan v. Tyler,

436 U.S. 499 (1978) .......................................... 5, 10

Miller v. United States,

357 U.S. 301 (1958) .............................................. 15

Minnesota v. Olson,

495 U.S. 91 (1990) .................................................. 5

Missouri v. McNeely,

569 U.S. 141 (2013) ................................................ 2

Mitchell v. Wisconsin,

588 U.S. 840 (2019) ................................................ 6

Monday v. Oullette,

118 F.3d 1099 (6th Cir. 1997) .............................. 21

New Jersey v. TLO,

469 U.S. 325 (1985) ................................................ 9

New York v. Burger,

482 U.S. 691 (1987) ................................................ 9

Payton v. New York,

445 U.S. 573 (1980) .............................................. 15

Pennington v. City of Rochester,

No. 13-CV-6304-FPG, 2020 WL

1151461 (W.D.N.Y. Mar. 9, 2020) ........................ 23

Riley v. California,

573 U.S. 373 (2014) ................................................ 1

vi

Sabbath v. United States,

391 U.S. 585 (1968) .............................................. 15

Terry v. Ohio,

392 U.S. 1 (1968) ................................................ 2, 8

Texas v. Brown,

460 U.S. 730 (1983) .............................................. 10

United States v. Cooks,

920 F.3d 735 (11th Cir. 2019) .............................. 20

United States v. Hastings,

246 F. Supp. 3d 1163 (E.D. Tex.

2017)................................................................ 11, 12

United States v. Hill,

649 F.3d 258 (4th Cir. 2011) ................................ 12

United States v. Holloway,

290 F.3d 1331 (11th Cir. 2002) ............................ 20

United States v. Jones,

565 U.S. 400 (2012) ................................................ 2

United States v. Martinez-Fuerte,

428 U.S. 543 (1976) ................................................ 8

United States v. Robinson,

414 U.S. 218 (1973) .............................................. 10

United States v. Santana,

427 U.S. 38 (1976) .................................................. 5

United States v. Timmann,

741 F.3d 1170 (11th Cir. 2013) ...................... 13, 14

vii

United States v. U.S. Dist. Ct. for the E.

Dist. Mich.,

407 U.S. 297 (1972) ................................................ 4

Warden v. Hayden,

387 U.S. 294 (1967) ................................................ 5

Welsh v. Wisconsin,

466 U.S. 740 (1984) .............................................. 12

Whren v. United States,

517 U.S. 806 (1996) .................................... 2, 10, 19

Williams v. Maurer,

9 F.4th 416 (6th Cir. 2021) ................................... 17

STATE CASES

Pennsylvania v. Edgin,

273 A.3d 573 (Pa. Super. Ct. 2022) ...................... 12

State v. Boggess,

340 N.W.2d 516 (Wis. 1983) ................................. 20

State v. Heard,

350 P.3d 1044 (Idaho 2015).................................. 22

CONSTITUTIONAL PROVISIONS

U.S. Const. Amendment IV ..........1-2, 4-5, 8-10, 15, 21

OTHER AUTHORITIES

Case of Richard Curtis, Fost. 135, 168

Eng. Rep. 67 (Crown 1757) .................................. 14

viii

Chief Joel F. Schults, The Peril and

Promise of Well-Being Checks, Nat’l

Police Ass’n, available at

https://tinyurl.com/ymcrmpeb .............................. 18

Dan A. Black et al., Criminal Charges,

Risk Assessment, and Violent

Recidivism in Cases of Domestic

Abuse (Nat’l Bureau of Econ. Rsch.,

Working Paper No. 30884, 2023);

https://tinyurl.com/2h9va9z6 ............................... 22

Julie A. Ward et al., National Burden of

Injury and Deaths from Shootings by

Police in the United States, 20152020, 4 Am. J. Pub. Health 387

(March 13, 2024),

https://doi.org/10.2105/AJPH.2023.30

7560 ....................................................................... 17

Lee v. Gansel, 1 Cowp. 1, 98 Eng. Rep.

935 (Crown 1774) ................................................. 15

Richard R. Johnson, Correlates of Rearrest Among Felony Domestic

Violence Probationers, 72 Fed.

Probation 3,

https://tinyurl.com/2yudkc9m .............................. 22

What Percentage of Americans Own

Guns?, Gallup (Nov. 13, 2020),

https://tinyurl.com/mr7bujj4 ................................ 16

1

INTEREST OF AMICI CURIAE

The National Association of Criminal Defense Lawyers (“NACDL”) is the preeminent national organization in the United States representing attorneys practicing in the field of criminal defense—including private criminal defense lawyers, public defenders, military defense counsel, law professors, and judges committed to ensuring fairness within America’s criminal

justice system. 1 The NACDL is a nonprofit, voluntary

professional bar association that frequently appears

as amicus curiae before this Court in cases raising issues of importance to criminal defendants and the defense bar.

NACDL and its many thousands of members have

an important interest in ensuring that the emergency

aid exception to the Fourth Amendment’s warrant requirement is applied in a manner that is consistent

with this Court’s precedents and that minimizes the

potential for abuse and the risk of dangerous confrontations between police officers and home occupants.

The American Civil Liberties Union (“ACLU”) is a

nationwide, non-profit, non-partisan organization

with more than 1.3 million members, founded in 1920

and dedicated to the principles of liberty and equality

enshrined in the Constitution. In support of those

principles, the ACLU has appeared before this Court

as counsel or amicus curiae in numerous Fourth

Amendment cases including Carpenter v. United

States, 585 U.S. 296 (2018) (counsel); Riley v. California, 573 U.S. 373 (2014) (amicus); Byrd v. United

No party authored this brief in whole or in part, and no person

or entity, other than the undersigned amici curiae, their members, or their counsel, has made a monetary contribution to the

preparation or submission of this brief.

1

2

States, 584 U.S. 395 (2018) (amicus); Missouri v.

McNeely, 569 U.S. 141 (2013) (counsel); United States

v. Jones, 565 U.S. 400 (2012) (amicus); Arizona v.

Gant, 556 U.S. 332 (2009) (amicus); Kyllo v. United

States, 533 U.S. 27 (2001) (amicus); Illinois v. Wardlow, 528 U.S. 119 (2000) (amicus); Whren v. United

States, 517 U.S. 806 (1996) (amicus); and Terry v.

Ohio, 392 U.S. 1 (1968) (amicus).

The ACLU of Montana is a non-profit, non-partisan

membership organization devoted to protecting civil

rights and liberties for all Montanans. It is the state

affiliate of the ACLU. For decades, the ACLU of Montana has litigated questions involving civil rights and

liberties in the state and federal courts, including

cases involving the Fourth Amendment.

INTRODUCTION AND SUMMARY OF

ARGUMENT

At the “very core” of the Fourth Amendment is “the

right of a man to retreat into his own home and there

be free.” Florida v. Jardines, 569 U.S. 1, 6 (2013) (citation omitted). This Court has thus never allowed

law enforcement officers to enter a private home—for

criminal investigations or non-criminal functions—

without probable cause to support entry.

The Court should reject Montana’s request to do so

for the first time here. The text of the Fourth Amendment makes the home the “first among equals,” id.,

and it expressly sets “probable cause” as the default

standard for investigatory actions, U.S. Const. amend.

IV. Accordingly, this Court’s longstanding jurisprudence generally requires officers to have a warrant

supported by probable cause to enter a home. This

Court has also permitted warrantless entry into a

home in certain exigent circumstances—for instance,

when in “hot pursuit” of a fleeing felon, when faced

3

with the imminent destruction of evidence, or when

emergency aid is required to protect an occupant’s

safety. See Brigham City v. Stuart, 547 U.S. 398, 403

(2006) (citation omitted). But even in those circumstances, this Court has never upheld a warrantless entry on less than probable cause.

Allowing home entries on a standard lower than

probable cause would create serious risks. First, the

existence of two different standards based on the officers’ underlying rationale would invite abuse. Officers

could use a thin “emergency aid” rationale to enter the

home and pursue otherwise inappropriate investigatory aims once inside, or as a post hoc justification for

an entry that lacked probable cause. Second, the increased number of home entries likely to flow from a

relaxed emergency-aid standard would exacerbate the

significant safety risks to officers and occupants when

officers enter a home.

On the other hand, requiring officers to have probable cause of an ongoing emergency or imminent

safety risk before entering a home will not hinder law

enforcement from responding effectively to legitimate

emergencies. The probable cause standard, familiar

to every officer in the country, is a “flexible, easily applied standard” that has proven administrable in the

field. Illinois v. Gates, 462 U.S. 213, 239 (1983). That

is true even where officers are performing functions

unrelated to criminal investigations. In most types of

emergencies, including fires, domestic abuse, medical

emergencies, and drug overdoses, this standard is easily satisfied.

In sum, this Court should hold that where police

officers or other state actors enter a home without a

warrant to render emergency aid or prevent imminent

injury to an occupant, they must have probable cause

4

to believe that a genuine emergency exists or that the

occupant’s safety is in imminent peril for the entry to

be reasonable under the Fourth Amendment.

ARGUMENT

I.

Protection of Individuals’ Privacy

Interests in Their Homes Is the

Paramount Concern of the Fourth

Amendment.

1. The Fourth Amendment guarantees the right

of the people “to be secure in their persons, houses, papers, and effects, against unreasonable searches and

seizures.” U.S. Const. amend. IV. It further provides

that “no Warrants shall issue, but upon probable

cause.” Id.

In light of this language, this Court has recognized that “physical entry of the home is the chief evil

against which the wording of the Fourth Amendment

is directed.” United States v. U.S. Dist. Ct. for the E.

Dist. Mich., 407 U.S. 297, 313 (1972). And because the

home is the “first among equals,” Jardines, 569 U.S. at

6, home entry by police officers typically requires a

warrant supported by probable cause, see Lange v.

California, 594 U.S. 295, 298 (2021). Indeed, a warrantless home entry is per se unreasonable, and therefore violates the Fourth Amendment, “unless the police can show that it falls within one of a carefully defined set of [exigent circumstance] exceptions” to the

warrant requirement. Coolidge v. New Hampshire,

403 U.S. 443, 474-475 (1971). Absent a warrant or a

showing of exigent circumstances, police officers may

not intrude upon the home “even when they have probable cause,” as such intrusions are “invasion[s] of the

sanctity of the home.” Collins v. Virginia, 584 U.S.

586, 596 (2018) (citation omitted).

5

These Fourth Amendment requirements apply

equally to state actors who are not engaged in criminal

law enforcement functions, such as health and safety

inspectors and firefighters. See Camara v. Mun. Ct. of

the City and Cnty. of San Francisco, 387 U.S. 523, 534

(1967) (health and safety inspectors); Michigan v. Tyler, 436 U.S. 499, 504 (1978) (firefighters).

2. In several circumstances, this Court has allowed

warrantless entry into a home based on “a genuine exigency.” Kentucky v. King, 563 U.S. 452, 470 (2011).

But the Court has never upheld a warrantless home

entry without probable cause to believe that the exigency existed. Cf. Minnesota v. Olson, 495 U.S. 91, 100

(1990) (affirming the Minnesota Supreme Court’s application of the proper legal standard where it held

that “there must be at least probable cause to believe

that one or more of the [exigent circumstance] factors

justifying the entry were present”). Nor has the Court

ever allowed state actors to enter a home for any other

reason based on less than probable cause.

a. Hot Pursuit. In United States v. Santana, 427

U.S. 38 (1976), the Court upheld a warrantless home

entry because officers were in “hot pursuit” of a suspect whom they believed was dealing drugs. Id. at 42.

Santana built upon the Court’s prior decision in Warden v. Hayden, 387 U.S. 294 (1967), which upheld entry into a private home to arrest a suspected armed

robber.

In both of these cases, the police had probable

cause to support their actions. In Santana, the police

had probable cause that the suspect was dealing

drugs. 427 U.S. at 42. The Santana Court noted that

officers in Hayden “had probable cause to believe” that

the suspect they were chasing had entered the home.

Id. And the Court has never suggested in any other

6

case that officers may pursue a fleeing suspect into a

home based on a mere reason, without probable cause,

to believe he may have committed a felony.

b. Destruction of Evidence. In Ker v. California,

374 U.S. 23 (1963), the Court upheld a warrantless

home entry based on the imminent destruction of evidence. The officers had probable cause to believe that

the suspect was in possession of narcotics “which could

be quickly and easily destroyed,” and the suspect’s furtive conduct alerted the officers to intervene. Id. at 4041; see also Mitchell v. Wisconsin, 588 U.S. 840, 850

(2019) (reasoning underlying imminent destruction of

evidence cases is that there is “no time to secure a warrant” (citation omitted)). Similarly, in Kentucky v.

King, 563 U.S. 452 (2011) the Court held that it was

reasonable “to dispense with the warrant requirement” where circumstances “led the officers to believe

that drug-related evidence was about to be destroyed.”

Id. at 457, 462. The Court allowed the entry because

there was no time to secure a warrant, but the Court

did not excuse the officers from meeting the probable

cause standard.

In each of these cases, officers observed or could infer with a fair degree of probability that evidence was

imminently going to be destroyed, and thus had probable cause to support their entry.

c. Emergency Aid. In Brigham City v. Stuart, 547

U.S. 398 (2006), this Court recognized that “[o]ne exigency obviating the requirement of a warrant is the

need to assist persons who are seriously injured or

threatened with such injury.” Id. at 403. In Michigan v. Fisher, 558 U.S. 45 (2009), the Court reaffirmed

the emergency aid exception. Id. at 47.

The Court did not use the term “probable cause” in

Brigham City and Fisher or expressly characterize the

7

level of suspicion necessary to justify entry. But the

facts of those cases make clear that officers had probable cause to believe an emergency was imminent or

ongoing. In Brigham City, officers responded to a

noise complaint about a loud house party. 547 U.S. at

401. They saw “through a screen door and windows—

an altercation taking place in the kitchen of the home,”

during which one partygoer struck another in the face.

Id. One officer testified that the person who was

struck spit blood into a nearby sink. Id. The officers’

personal observations of an ongoing fight were more

than sufficient “‘to warrant a man of reasonable caution in the belief that’ an [emergency] has been or is”

occurring. Brinegar v. United States, 338 U.S. 160,

175-176 (1949) (citation omitted).

In Fisher, officers responded to a call about a disturbance in a residential neighborhood. 558 U.S. at

45. When they arrived, officers saw a smashed pickup

truck, damaged fenceposts, three broken house windows, and blood on a truck, on the clothes inside, and

on the doors of the house. Id. at 45-46. Officers also

saw a man inside the house screaming and throwing

things and observed a cut on the man’s hand. Id. at

46. Because officers “encountered a tumultuous situation in the house,” found “signs of a recent injury,”

and “could see violent behavior inside,” id. at 48, they

undoubtedly had probable cause to enter the house to

prevent injury to the screaming individual or someone

else in the home.

3. This Court has permitted searches and seizures

based on less than probable cause only where individuals have reduced expectations of privacy or where the

intrusion is limited in scope.

For example, vehicles at a permanent, fixed checkpoint near the border may be briefly inspected without

8

suspicion because “one’s expectation of privacy in an

automobile . . . [is] significantly different from the traditional expectation of privacy and freedom in one’s

residence,” United States v. Martinez-Fuerte, 428 U.S.

543, 561 (1976), and the government’s interest in immigration enforcement is high, id. at 557. In contrast

to government intrusions into “the sanctity of private

dwellings,” the level of “intrusion on Fourth Amendment interests” involved in brief visual inspections of

vehicles at permanent checkpoints near the border is

“quite limited.” Id. at 557-558. The same is true of sobriety checkpoints. See Michigan Dep’t of State Police v. Sitz, 496 U.S. 444, 451-452 (1990); see also Ferguson v. City of Charleston, 532 U.S. 67, 83 n.21 (2001)

(distinguishing Martinez-Fuerte and Sitz on the basis

that those cases did not involve an “intrusive search

of . . . the home” (citation omitted)).

Officers may also briefly stop and frisk an individual on a public street based upon a reasonable suspicion of that person being “armed and presently dangerous to the officer or to others.” Terry v. Ohio, 392

U.S. 1, 24 (1968). During a stop-and-frisk, the Fourth

Amendment’s demand of reasonableness is satisfied

even where officers do not yet have probable cause

that a crime has occurred because the “limited search

for weapons” is only a “brief, though far from inconsiderable, intrusion upon the sanctity of the person” that

amounts to “less than a full search.” Id. at 26.

Likewise, the Court has allowed searches based on

“special needs” only where individuals have diminished expectations of privacy. For example, the expectation of privacy in “commercial premises” is “different

from, and indeed less than, a similar expectation of

privacy in an individual’s home” and is “particularly

attenuated in commercial property employed in

9

‘closely regulated’ industries.” New York v. Burger,

482 U.S. 691, 700 (1987); see also New Jersey v. TLO,

469 U.S. 325 (1985) (searches in schools allowed based

on reasonable suspicion); Florence v. Bd. of Chosen

Freeholders, 566 U.S. 318 (2012) (strip searches at

jails during booking procedures allowed without suspicion).

II.

Allowing Officers to Enter Homes to

Render Emergency Aid On Less Than

Probable Cause Would Create

Opportunities for Abuse and Significant

Safety Risks for Officers and Occupants.

In situations where officers suspect that an individual inside a home requires aid or is at imminent risk

of injury, officers must balance the importance of swift

action against the privacy and security interests embedded in the Fourth Amendment. The probable

cause standard is the “best compromise . . . for accommodating” such “opposing interests.” Brinegar, 338

U.S. at 176. A lower standard of proof would create

incentives for officers to use the emergency aid exception to gain entry into homes for other law enforcement purposes or to rationalize unlawful entries after

the fact. It would also unnecessarily expose officers

and home occupants to the significant safety risks inherent in residential encounters.

A.

Opportunities For Abuse

1. Because entry into the home requires probable

cause in all other circumstances, allowing warrantless

home entries on less than probable cause to render

emergency aid would create an incentive for officers to

seek reasons to enter based on the emergency aid exception in order to engage in other law enforcement

activity. A series of other established doctrines combines to exacerbate this risk.

10

To start, officers’ subjective motives do not matter

when assessing whether a Fourth Amendment search

or seizure is reasonable. See Whren v. United States,

517 U.S. 806, 813 (1996). In Fisher, the Court confirmed that in the emergency aid context the Fourth

Amendment reasonableness analysis “does not depend

on the officers’ subjective intent,” but asks only

whether they had “an objectively reasonable basis for

believing . . . that a person within [the house] is in need

of immediate aid.” 558 U.S. at 47 (citation and internal quotation marks omitted). Because officers’ intent

is constitutionally irrelevant, their entry into the

home is lawful even where the reason justifying the

entry is pretextual.

Once lawfully inside the home—even with less

than probable cause to justify the entry—other Fourth

Amendment rules would permit officers to engage in a

much wider investigation. An officer may seize evidence of criminal wrongdoing in plain view. See Coolidge, 403 U.S. at 465-466; Texas v. Brown, 460 U.S.

730, 737-739 (1983) (plurality opinion); see also Michigan v. Tyler, 436 U.S. at 509 (“[O]nce in a building [to

put out blazes], firefighters may seize evidence of arson that is in plain view.”). If officers develop probable

cause for an arrest while inside the home, they may

also search the person and the immediate area as part

of a search incident to arrest, see United States v. Robinson, 414 U.S. 218 (1973), or other rooms in the home

as part of a protective sweep, see Maryland v. Buie,

494 U.S. 325 (1990). 2

Although Buie allows officers to perform protective sweeps

based on reasonable suspicion of threats to their safety, their initial entry into the home must have been supported by probable

cause. See Buie, 494 U.S. at 330 (police could enter and search

(footnote continued)

2

11

2. The combination of these doctrines would open

the door to officers who lack probable cause to look for

some lesser suspicion of an emergency to justify entry.

If officers need only some objective reason, but not

probable cause, to believe there may be an emergency

inside a home, they could enter the home based on ambiguous cues—the sound of a loud bang or pop, the

smell of smoke (maybe from a backyard barbecue or

fire pit, or maybe not), or raised voices; the sight of a

broken window or an occupant carrying a gun; or a call

reporting a disturbance next door. In that world, officers could use ambiguous indicia of an emergency to

enter the home of an occupant they wish to investigate

for other reasons, criminal or not.

Cases around the country already show how home

entries under the emergency aid exception can expand

into broader investigations.

a. In United States v. Hastings, 246 F. Supp. 3d

1163 (E.D. Tex. 2017), the Secret Service had issued a

“nationwide BOLO” for the defendant and “wanted the

officers to locate that individual and take him into custody.” Id. at 1171. Officers then breached the defendant’s hotel room without an arrest or search warrant

on the ground that the defendant was “possibly suicidal” and presented a risk of “suicide by cop.” Id. at

1176. But at the time of the officers’ entry, the defendant “was not displaying any increasing mental instability” and officers did not “communicat[e] with him or

gaug[e] his mental state.” Id. at 1167. After justifying

entry on the ground that the defendant was suicidal,

for suspect in a home “based on the authority of the arrest warrant”).

12

officers collected evidence in the hotel room and ultimately charged the defendant using the evidence they

discovered. Id. at 1174.

b. In Pennsylvania v. Edgin, 273 A.3d 573 (Pa. Super. Ct. 2022), officers relied on the emergency exception to enter a home through an “unlocked, sliding

door in the back of the house” purportedly to assist an

intoxicated individual within. Id. at 583, 585. Once

inside, the officers proceeded to “detain, mirandize,

and interrogate” the occupant on suspicion of drunk

driving, id. at 586, even though this Court has disallowed “warrantless home arrest[s]” for that offense,

Welsh v. Wisconsin, 466 U.S. 740, 754 (1984). By relying on the emergency exception, the police quite literally used a backdoor for their investigation.

c. In United States v. Hill, 649 F.3d 258 (4th Cir.

2011), officers searching for a suspect for whom they

had an arrest warrant entered the suspect’s girlfriend’s home, thinking that the girlfriend’s sister

might be “facing some safety issues.” Id. at 261, 265.

The officers “expressed concern” after seeing an old

doorframe was slightly damaged and knocking and

getting no response, even though they heard the television on in the living room. Id. at 265, 266. Once

inside to purportedly provide emergency aid, officers

found the target of their arrest warrant, conducted a

protective sweep, and uncovered evidence of criminal

wrongdoing in plain view. Id. at 261-262. The sister

was not in the house.

These examples show that officers have and will

use the emergency exception as a pretext to enter

homes for other investigative purposes. Permitting

emergency entries on suspicion less than probable

cause will only increase the number of such pretextual

entries.

13

3. Similarly, a standard lower than probable cause

provides a potential post hoc rationale to justify otherwise unlawful entries.

a. For example, in McInerney v. King, 791 F.3d

1224 (10th Cir. 2015), an officer investigating a complaint that a woman shoved her ex-husband’s girlfriend went to the woman’s house to serve her with a

summons for harassment. Id. at 1227. There he observed “two open front windows,” a front door “open

about six inches,” and an open garage door, but he

never told the senior officer who arrived shortly after

him that he thought there was an ongoing emergency.

Id. at 1228. Both officers proceeded to enter the home

under the guise of a welfare check, guns drawn. Id. at

1228. Inside, they found a woman “partially dressed,”

soundly asleep in her bed, and started “screaming and

yelling at her while shining a flashlight in her eyes.”

Id. at 1228-1229. Only on summary judgment in the

woman’s section 1983 claim did the officer introduce,

for the first time, facts related to the woman’s history

of drug abuse to attempt to show that a reasonable officer could have believed there was a need to assure

the safety of those in the home. Id. at 1232-1233.

b. In United States v. Timmann, 741 F.3d 1170

(11th Cir. 2013), an officer responded to a “service call”

from a woman who found a bullet hole in the wall between her bedroom and the apartment unit next door.

Id. at 1173. Officers knocked several times on the unit

next door, received no response, and confirmed that no

calls reporting gunshots or disturbances at the apartment had been made in the preceding days. Id. at

1174. Officers returned the next day to “further investigate” and entered the unit that they suspected was

the origin of the bullet. Id. at 1174-1175. They found

“no one present and no signs of injury,” but found guns

14

in plain view and charged the resident as a felon in

possession. Id. at 1175-1176. Though there were no

signs of an ongoing emergency when officers entered

the apartment, the district court found the entry lawful under the emergency aid exception because of the

presence of the bullet hole. Id. at 1177, 1180-1181.

The Eleventh Circuit reversed on appeal.

In sum, a lower standard of proof for the emergency

aid exception is ripe for abuse, either as pretext for

other investigatory purposes or as a retrospective excuse to justify unlawful entry.

B.

Significant Safety Risks

Law enforcement entries into people’s homes are

inherently volatile and dangerous, regardless of the

type of exigency involved. Requiring probable cause

appropriately balances these real risks for officers and

home occupants with a legitimate need to render

emergency aid.

1. Since before the Founding, courts have recognized the dangers associated with warrantless or unexpected home entries. Famously, a common law English court once reviewed the murder conviction of a

man who killed a Crown officer and argued that the

killing was justified because the officer had failed to

announce himself. See Case of Richard Curtis, Fost.

135, 168 Eng. Rep. 67 (Crown 1757). The Court remarked that officers effectuating a home entry must

provide notice “that the officer cometh not as a mere

trespasser, but claiming to act under a proper authority.” Id. at 68. Likewise, Lord Mansfield once remarked that allowing bailiffs to make unannounced

home entries “would leave the family within, naked

and exposed to thieves and robbers. It is much better

therefore, says the law, that you should wait for another opportunity, than do an act of violence, which

15

may probably be attended with such dangerous consequence.” Lee v. Gansel, 1 Cowp. 1, 7, 98 Eng. Rep. 935,

938 (Crown 1774).

Following this common-law tradition, this Court’s

Fourth Amendment jurisprudence recognizes that a

“crucial factor” in determining the reasonableness of a

government intrusion is whether the intrusion may

“threaten the safety or health of [an] individual” or

otherwise present a “physical danger.” Maryland v.

King, 569 U.S. 435, 464 (2013) (citation omitted). On

numerous occasions, this Court has considered the

dangers inherent in law enforcement home encounters

and crafted rules designed to mitigate them.

For example, this Court struck down the

“longstanding, widespread practice” of warrantless entries into homes to conduct routine arrests, in part because some common-law sources highlighted “substantial risks in proceeding without [a warrant].” Payton

v. New York, 445 U.S. 573, 596, 600 (1980). The Court

has also recognized that requiring officers to knock

and announce prior to entry protects “human life and

limb, because an unannounced entry may provoke violence in supposed self-defense by the surprised resident.” Hudson v. Michigan, 547 U.S. 586, 594 (2006);

see also Sabbath v. United States, 391 U.S. 585, 589

(1968) (“[P]rior notice of authority and purpose” also

“safeguard[s] officers”).

Those risks are particularly amplified because

many people keep firearms in their homes for self-defense. See Dist. of Columbia v. Heller, 554 U.S. 570,

629 (2008) (observing that “handguns are the most

popular weapon chosen by Americans for self-defense

in the home”); Miller v. United States, 357 U.S. 301,

313 n.12 (1958) (requiring notice as a “safeguard for

the police themselves who might be mistaken for

16

prowlers and be shot down by a fearful householder”);

What Percentage of Americans Own Guns?, Gallup

(Nov. 13, 2020), https://tinyurl.com/mr7bujj4 (last accessed August 4, 2025) (44% of Americans reported living in a gun-owning household).

2. Officers entering a home under exigent circumstances—especially where officers believe that someone’s life or well-being is on the line—may be required

to make fast-moving decisions based on incomplete information, or information that may prove unreliable or

outright false. See Pet’r Br. 41-42 (discussing the risks

of “swatting,” in which someone reports a false emergency at another’s home to create an armed police response as a way to harass and threaten the home occupant). If police are mistaken about the existence of

an emergency requiring aid, or if they are right but

their actions escalate the situation, a sudden entry exacerbates risks that an occupant may mistake officers

for home invaders, that officers may encounter a violent response, or that officers may cause damage to

property.

Cases from around the country highlight the serious, and potentially deadly, consequences attendant

with these risks.

a. In Est. of Chamberlain v. City of White Plains,

960 F.3d 100 (2d Cir. 2020), a man accidentally activated his Life Aid medical button, prompting an operator to summon emergency services personnel. Id. at

101. When officers knocked on the door, the man repeatedly told them that he did not need their help. Id.

at 102. Officers nevertheless tried to forcibly enter the

man’s home. Id. The man grew agitated and pulled

out a knife. Id. at 103. After an hour-long standoff,

officers entered the home, tased, shot, and killed the

man. Id.

17

b. In Williams v. Maurer, 9 F.4th 416 (6th Cir.

2021), officers received an anonymous 911 call about a

break-in. Officers arrived at the apartment number

the tipster provided but heard nothing inside. Id. at

423. The tipster called back and stated she was unsure of the right apartment. Id. After eight minutes

and a brief exchange once an occupant opened the

door, the officers rushed inside because “we got exigent

circumstances.” Id. at 423-425. The officers tackled

both occupants, arrested one for obstruction of police,

“left a permanent scar” on the other. Id. at 425.

c. In Luethje v. Kyle, 131 F.4th 1179 (10th Cir.

2025), officers received a 911 call about a break-in

where the suspect had fled the scene. Deputies “observed nothing further” inside or outside the home

other than a broken window with the screen still in

place. Id. at 1191. The officers immediately removed

the screen and sent in a canine to “bite whomever it

found inside the residence.” Id. at 1185. The canine

repeatedly bit a sleeping resident, who had broken the

window to get inside. Id. at 1186. After the resident

was transported to the hospital, officers “conducted a

thorough search,” finding “no other person” inside and

“no evidence of a crime.” Id.

d. Welfare checks in particular are among the most

dangerous encounters between individuals and law

enforcement. One study showed that “[w]ell-being

checks were 74% more likely to be associated with fatal injury” than other incidents of officer-involved

shootings, “despite not explicitly or necessarily involving pre-encounter threats of harm.” Julie A. Ward et

al., National Burden of Injury and Deaths from Shootings by Police in the United States, 2015-2020, 4 Am.

J. Pub. Health 387, 393 (March 13, 2024),

18

https://doi.org/10.2105/AJPH.2023.307560 (last accessed August 4, 2025). To reduce these risks, many

local governments recognize that health professionals

may best handle some emergencies without police involvement, while others have adopted a model of coresponder programs where a social worker accompanies officers during welfare checks on individuals with

mental illness. See Chief Joel F. Schults, The Peril

and Promise of Well-Being Checks, Nat’l Police Ass’n,

available at https://tinyurl.com/ymcrmpeb (last accessed August 4, 2025). But smaller and less resourced police forces may not have such programs, increasing the risk of violent altercations. Id.

For example, in Buchanan v. Maine, 469 F.3d 158

(1st Cir. 2006), two officers in rural Maine responded

to a call for a welfare check from the neighbor of a man

with a long history of mental illness. Id. at 164. The

man screamed at the officers, threw liquid appearing

to be liquor at one of them through an open window,

and punched open another window, bloodying his

hand. Id. at 165. Observing the man’s erratic and

dangerous behavior, one of the officers entered the

home “to put [the man] into protective custody and

have him evaluated.” Id. Once inside, the man

grabbed a knife and stabbed the officer. Id. at 166.

The officer’s partner rushed inside and fatally shot the

mentally unstable man. Id.

Given the risks of home entries, and particularly

welfare check entries, the probable cause standard appropriately balances risks and benefits. Because the

probable cause standard demands that officers

“search[] for the clues and corroboration” that a genuine exigency exists, Buckley v. Fitzsimmons, 509 U.S.

259, 273 (1993), it encourages officers to seek addi-

19

tional assurances that aid is probably required to justify the risks to both officers and occupants, rather

than entering a home based on mere reason to think

that aid might be needed.

III.

Requiring Probable Cause of Imminent

Injury Will Not Hinder Law Enforcement

From Rendering Necessary Aid in

Emergency Situations.

Requiring officers to have probable cause to enter

a home will not prevent first responders from responding appropriately to emergencies. The probable cause

standard is an objective standard familiar to officers,

and, in most common types of emergencies, officers or

other first responders have more than enough evidence of an ongoing emergency or need for aid to meet

the probable cause bar.

A.

Probable Cause Is an Objective,

Administrable Standard Familiar to

Officers in the Field.

Probable cause is a “flexible, easily applied standard” that has proven administrable in the field. Gates,

462 U.S. at 239. It is based on an objective view of the

circumstances surrounding an officer’s actions. Id.;

see Whren, 517 U.S. at 813. And it is a “practical, nontechnical conception that deals with the factual and

practical considerations of everyday life.” Maryland v.

Pringle, 540 U.S. 366, 370 (2003) (citation omitted).

The Court has clarified that all that probable cause requires “is the kind of ‘fair probability’ on which ‘reasonable and prudent [people,] not legal technicians,

act.’” Florida v. Harris, 568 U.S. 237, 244 (2013) (quoting Gates, 462 U.S. at 238, 231).

While the probable cause standard is most familiar

to officers in the criminal investigations context, the

20

Court has also applied the concept of probable cause

in non-criminal contexts. For health and safety inspections, for instance, the reasonableness of a particular inspection depends on “whether there is probable

cause to issue a warrant for that inspection.” Camara,

387 U.S. at 535. In such contexts, “the facts that

would justify an inference of ‘probable cause’ to make

an inspection are clearly different from those that

would justify such an inference where a criminal investigation has been undertaken.” Id. at 538. But the

probable cause analysis still depends on contextual

cues related to the underlying goal of the search. Id.

at 538-539 (listing “the passage of time, the nature of

the building . . ., [and] the condition of the neighborhood” as exemplary factors in the health and safety

context).

In short, “[t]he test of ‘probable

cause’ . . . can take into account the nature of the

search that is being sought.” Id. at 538.

In the emergency aid context, probable cause has

been the rule in many federal and state courts for as

long as four decades. See State v. Boggess, 340 N.W.2d

516, 523 (Wis. 1983) (applying the Gates “totality of

the circumstances” probable cause analysis to an

emergency aid entry and assessing “the presence of detail in the information [provided to police], and corroboration of details of an informant’s tip by independent

police work”); Kerman v. City of New York, 261 F.3d

229, 236 (2d Cir. 2001) (officers lacked probable cause

for warrantless entry because they “had no corroborating evidence of the alleged danger [from an anonymous call] to establish reliability”); United States v.

Holloway, 290 F.3d 1331, 1338 (11th Cir. 2002) (probable cause for entry existed “based on the information

conveyed by the 911 caller and the personal observations of the officers”); United States v. Cooks, 920 F.3d

735, 745 (11th Cir. 2019) (Newsom, J.) (“[R]easonable

21

inferences based on both knowns and known unknowns—rather than concrete evidence of harm

alone—can establish probable cause to believe that an

innocent is in danger and in need of immediate aid.”

(citation omitted)).

Lower courts have also applied the probable cause

standard to mental health seizures. See, e.g., Monday v. Oullette, 118 F.3d 1099, 1102 (6th Cir. 1997) (remarking that if “a dangerous mental condition is analogized to the role of criminal activity in traditional

Fourth Amendment analysis, a showing of probable

cause . . . requires only a ‘probability or substantial

chance’ of dangerous behavior”); Cantrell v. City of

Murphy, 666 F.3d 911, 923 (5th Cir. 2012).

If this Court adopts the probable cause standard

for situations where emergency aid may be warranted,

officers can rely on a developed body of case law to

guide their decision-making.

B.

A Probable Cause Standard Will Not

Prevent Officers or Other First

Responders From Entering the

Home to Address Emergencies.

Requiring state actors to have probable cause before entering a home to render emergency aid will not

hinder law enforcement and other first responders

from effective emergency response. In the most common types of home emergencies—fires, domestic

abuse, overdoses, or medical emergencies, especially

among the elderly—the probable cause bar is not difficult for first responders to meet.

1. In the context of fires, this Court recognized decades ago that a “burning building of course creates an

exigency that justifies a warrantless entry by fire officials to fight the blaze.” Michigan v. Clifford, 464 U.S.

22

287, 293 (1984). When firefighters are dispatched to a

potential fire, the presence of smoke or flames provides obvious probable cause to justify their entry under the emergency aid exception.

As for officers responding to domestic violence

calls, this Court observed in Georgia v. Randolph, 547

U.S. 103 (2006), that “[n]o question . . . reasonably

could be [raised] about the authority of the police to

enter a dwelling to protect a resident from domestic

violence; so long as they have good reason to believe

such a threat exists.” Id. at 118; see State v. Heard,

350 P.3d 1044 (Idaho 2015) (upholding warrantless

entry into hotel room where hotel clerk reported loud

arguing and fighting between a man and a woman and

officers saw the woman motionless and barely responsive in the room upon arrival).

Many domestic violence calls come from the victim

or other individuals within the home, or from neighbors, family, or friends with specific and reliable information. Officers can corroborate this information with

their own observations or by looking in their database,

as many domestic abusers are repeat offenders known

to law enforcement. See, e.g., Dan A. Black et al.,

Criminal Charges, Risk Assessment, and Violent Recidivism in Cases of Domestic Abuse 4-5 (Nat’l Bureau

of Econ. Rsch., Working Paper No. 30884, 2023);

https://tinyurl.com/2h9va9z6 (last accessed August 4,

2025); Richard R. Johnson, Correlates of Re-arrest

Among Felony Domestic Violence Probationers, 72 Fed.

Probation 3, https://tinyurl.com/2yudkc9m (last accessed August 4, 2025).

In drug overdose cases, a call from a reliable source

reporting that the occupant is likely overdosing on

drugs would be enough to meet the probable cause bar;

or where the source is anonymous or its reliability is

23

uncertain, police can gather information to meet the

probable cause bar to justify their entry.

For example, in Pennington v. City of Rochester,

No. 13-CV-6304-FPG, 2020 WL 1151461 (W.D.N.Y.

Mar. 9, 2020), two officers went to a woman’s home to

perform a welfare check. Id. at *2. When officers arrived, they noticed an open inner door and saw the

woman through the window, lying motionless on the

couch with two cans of beer next to her. Id. When the

woman did not respond to officers’ knocks and shouts,

they entered, concerned that she was suffering from

alcohol poisoning or had overdosed. Id. The court upheld the entry under the Second Circuit’s probable

cause standard. Id. at *3.

Home welfare checks can also meet the probable

cause standard. For example, in Gaetjens v. City of

Loves Park, 4 F.4th 487 (7th Cir. 2021), cert. denied,

142 S. Ct. 1765 (2022), a woman’s doctor and neighbor

were unable to reach her after her doctor recommended she go to the hospital for high blood pressure.

The neighbor—listed by the woman as her emergency

contact—called the police for a welfare check. Id. at

493. Even though the officers did not “see anyone inside,” they observed “packages on the porch, untended

garbage, and a full mailbox.” Id. at 490. The officers

interviewed the neighbor again before asking for a

spare key. Id. at 490. The Seventh Circuit found that

this welfare check “f[ell] into the heartland of emergency-aid situations.” Id. at 493.

This answers the question raised during oral argument in Caniglia v. Strom, 593 U.S. 194 (2021), where

the Chief Justice asked whether police could enter the

home of an elderly woman about whom neighbors had

called because the woman had uncharacteristically

24

not shown up to dinner. See Transcript of Oral Argument at 6:13-7:2, Caniglia v. Strom 593 U.S. 194

(2021) (No. 20-157). In the Chief Justice’s hypothetical, the neighbors did not see the woman leave and

could not reach her or her family members by phone.

While there will always be cases on the edge of the

probable cause analysis, most courts would likely find

those facts sufficient to show a “fair probability” that

the elderly woman needed emergency aid. In any close

cases, the probable cause standard would encourage

officers to corroborate their beliefs, as they did in Gaetjens, before entering the home. Furthermore, under

the current law on qualified immunity, an officer

would be protected from liability unless it is clearly established that his actions are unlawful.

2. The facts of this case are consistent with—and

indeed illuminate—these principles. Here, there was

no probable cause to justify the officers’ warrantless

entry—not because of a lack of information, but because the police had more than the typical amount of

information. The officers’ knowledge about the petitioner’s past interactions with law enforcement, along

with what they observed outside the home, undermined the report they had received about a possible

suicide attempt. In fact, the responding officers stated

it was unlikely that petitioner required immediate aid

and more likely that petitioner was waiting inside for

officers to enter in hopes of inviting a violent response.

See Pet. App. 28a-30a (dissenting opinion of Justice

McKinnon). Based on these facts, officers should have

known that not only was there no cause for entry, but

that their entry itself would create the risk for injury.

In other circumstances, a credible report of a suspected suicide within a home, by itself, might well

amount to probable cause to justify entry. But here,

25

the totality of the circumstances showed that petitioner did not require emergency aid.

All told, drawing a bright-line probable cause rule

will not prevent officers or state actors from rendering

necessary aid in emergency situations. Officers and

first responders will satisfy probable cause for most

common emergency aid situations. The Court should

not dilute the long-established probable cause standard to justify entry where officers or other state actors

either lack information to reach probable cause or, as

here, have important information that undermines the

reliability of information that might, absent that

broader context, amount to probable cause.

CONCLUSION

The judgment below should be reversed.

Respectfully submitted,

CECILLIA D. WANG

YASMIN CADER

BRANDON BUSKEY

AMERICAN CIVIL LIBERTIES

UNION FOUNDATION

125 Broad Street, 18th Floor

New York, NY 10004

ALEX RATE

AMERICAN CIVIL LIBERTIES

UNION FOUNDATION OF MONTANA

P.O. Box 1968

Missoula, MT 59806

August 5, 2025

DANIEL B. LEVIN

Counsel of Record

COLIN A. DEVINE

QIAN ZHE (DANNY) ZHANG

MUNGER, TOLLES & OLSON LLP

350 S. Grand Avenue, 50th Floor

Los Angeles, CA 90071

daniel.levin@mto.com

(213) 683-9100

JEFFREY L. FISHER

Co-Chair, AMICUS COMMITTEE

NATIONAL ASSOCIATION OF

CRIMINAL DEFENSE LAWYERS

1660 L Street, NW, 12th Floor

Washington, DC 20036