Module 1: Fourth Amendment — Search & Seizure

The core Fourth Amendment rules below are the same body of law taught in our Law Enforcement course’s Module 1 — this module doesn’t re-teach that lecture from scratch. Instead, it moves quickly through the same rules and then goes somewhere that course doesn’t: how these rules actually get litigated — through a motion to suppress, with real burdens of proof and real appellate standards of review. If any rule below feels compressed, that companion module has the fuller lecture treatment.

Under Katz v. United States, a search occurs when government conduct intrudes on a reasonable expectation of privacy. A seizure of a person occurs when a reasonable person would not feel free to leave or end the encounter.

Katz v. United States, 389 U.S. 347 (1967), is the case the test is named for, and it replaced an older, much narrower approach that tied Fourth Amendment protection to physical trespass onto property. FBI agents, without a warrant, attached a listening device to the outside of a public telephone booth to record a man’s side of his calls, which he believed were private; because the agents never physically entered the booth, the government argued no “search” had occurred at all under the old trespass-based framework. The Supreme Court rejected that framework, holding that the Fourth Amendment “protects people, not places” — what a person knowingly exposes to the public isn’t protected, but what they seek to preserve as private, even in a public space like a phone booth, may be. Justice Harlan’s concurrence supplied the two-part test still used today: did the person have an actual, subjective expectation of privacy, and is that expectation one society recognizes as objectively reasonable. The case remains the reason Fourth Amendment analysis today asks about privacy expectations rather than physical intrusion onto property alone.

Example. An officer looking over a low fence from a public sidewalk generally isn’t conducting a “search” — there’s no reasonable expectation of privacy in what’s visible from a public vantage point.

Reasonable suspicion requires specific, articulable facts suggesting criminal activity. A Terry stop is a brief investigative detention based on this standard, and any accompanying frisk needs its own separate justification — the frisk is for officer/public safety, not a license to search for evidence.

Terry v. Ohio, 392 U.S. 1 (1968), is the case that created this entire framework, allowing police to act on less than probable cause for the first time. A veteran police detective watched two men repeatedly walk past and peer into the same store window, confer with each other, and walk the same route again several times — a pattern he recognized, from his own experience, as consistent with casing a location for a robbery. He approached, identified himself, and, when the men gave mumbled answers, patted down their outer clothing and found a concealed handgun on one of them. The defendant argued the officer needed the full probable cause an arrest would require before conducting any pat-down at all. The Supreme Court disagreed, holding that a brief investigative stop, and a protective frisk for weapons, are far less intrusive than a full arrest and search, and can be justified by the lesser standard of specific, articulable facts supporting a reasonable suspicion of criminal activity — plus, for the frisk specifically, a genuine, separate concern for officer safety. The case remains the foundational authority creating an entire intermediate category of police-citizen encounters between a consensual conversation and a full arrest.

Example. A stop based on a witness description plus flight when approached can be valid; a frisk during that stop still needs its own separate basis to believe the person is armed.

Probable cause — facts sufficient to lead a reasonable person to believe a crime occurred — is generally required for an arrest or a search warrant, which must rest on a sworn affidavit reviewed by a neutral magistrate and describe the place and items with particularity.

Illinois v. Gates, 462 U.S. 213 (1983), replaced a rigid, two-part test for evaluating an informant’s tip with the flexible standard courts use today. Police received an anonymous letter predicting, in detail, a couple’s specific plan to drive to Florida, load a car with drugs, and drive back — and when independent police surveillance corroborated several of those predicted details, a magistrate issued a search warrant. Under the older Aguilar-Spinelli test, a warrant based on an informant’s tip required separately establishing both the informant’s basis of knowledge and their veracity — two rigid, independent prongs, each of which had to be satisfied on its own. The Supreme Court abandoned that rigid two-pronged approach in favor of a totality of the circumstances test: a weakness in one factor (like an anonymous tipster’s unknown reliability) can be offset by strength in another (like detailed predictions later corroborated by independent police work), so long as the magistrate’s overall, practical, common-sense judgment is that probable cause exists. The case remains the governing standard for how magistrates and reviewing courts evaluate probable cause based on an informant’s tip today.

Real, well-defined exceptions let officers act without a warrant: plain view, search incident to arrest (narrowed for vehicles by Arizona v. Gant), the automobile exception, exigent circumstances, and consent — each with its own specific, limited requirements.

Arizona v. Gant, 556 U.S. 332 (2009), is the case that narrowed the search-incident-to-arrest exception for vehicles, and its facts show exactly what changed. A man was arrested for driving with a suspended license, handcuffed, and secured in the back of a patrol car — with no way to reach his own vehicle at all — and officers then searched the car anyway, finding cocaine in a jacket pocket. Under the older, broader reading of the exception, courts had generally allowed a search of a vehicle’s passenger compartment incident to any recent occupant’s arrest, regardless of whether the arrestee could still reach it. The Supreme Court held that rationale no longer applied once the arrestee was handcuffed and locked in a patrol car — the exception’s real justifications (officer safety, and preventing the arrestee from destroying evidence) simply don’t exist once the arrestee has no possible access to the vehicle. Gant narrowed the exception to two situations: the arrestee is unsecured and within reaching distance of the vehicle at the time of the search, or it’s reasonable to believe the vehicle contains evidence of the specific offense of arrest. The case is why this exception is now considerably narrower for vehicle searches than it once was.

All of the doctrine above matters, procedurally, because of one mechanism: the motion to suppress, generally filed pretrial under Federal Rule of Criminal Procedure 12 or a state equivalent. Raise it too late — mid-trial, or for the first time on appeal — and it’s generally forfeited, subject at best to a much harder plain-error standard. At the hearing itself, the defendant generally must establish standing and a colorable claim that a search occurred; the burden then typically shifts to the government to justify the search as reasonable. On appeal, courts generally review factual findings for clear error but review the ultimate constitutional conclusion de novo.

Rakas v. Illinois, 439 U.S. 128 (1978), is the case governing that first standing hurdle, and it rejected a broader theory of who may even bring a suppression motion. Passengers in a car that was stopped and searched, revealing a rifle and ammunition hidden under a seat, sought to suppress that evidence, arguing that anyone legitimately present in a vehicle when it’s illegally searched should be able to challenge the search. The Supreme Court disagreed, holding that Fourth Amendment rights are personal — a defendant can challenge a search only by showing their own legitimate, reasonable expectation of privacy in the specific place searched, not merely that the search happened to turn up evidence used against them. Mere presence in someone else’s car, without some personal connection to the area searched (like being the owner, or having a right to exclude others from it), doesn’t supply that expectation. The case is the reason a defendant seeking to suppress evidence always has to clear this threshold, case-specific hurdle first — showing it was their own reasonable expectation of privacy that the government intruded on — before the court even reaches whether the search itself was constitutional.

Example. Defense counsel who spots a genuine Fourth Amendment problem but waits until closing argument to raise it has very likely forfeited the issue entirely — compare that to counsel who files a timely, specific pretrial motion, forcing the government to justify the search on the record.