Module 1: Police Authority & the Fourth Amendment

An officer’s authority derives from the state’s general police power to protect public health, safety, and welfare — real, substantial authority, but bounded at every turn by the Constitution, especially the Fourth Amendment’s protections against unreasonable searches and seizures.

This general “police power” is worth understanding precisely because of where it comes from, and where it doesn’t. Unlike the federal government, which can act only through powers actually enumerated in the Constitution, state and local governments hold a broad, general power to legislate and act for public health, safety, morals, and welfare — a power the states never gave up when they ratified the Constitution, reserved to them under the Tenth Amendment. That’s why most day-to-day policing (traffic enforcement, disorderly conduct, local ordinances) rests on state and local authority, while federal law enforcement agencies generally can act only within the narrower, specific bounds of an actual enumerated federal power (like regulating interstate commerce or enforcing federal criminal statutes). Both state and federal authority, however differently sourced, run into the exact same constitutional ceiling: however broad the underlying power to act, the Fourth Amendment’s reasonableness requirement applies equally, regardless of which government is doing the searching or seizing.

Example. An officer has broad authority to investigate a reported crime, question witnesses, and patrol public areas — but that same authority doesn’t extend to searching a person’s home without either a warrant or a recognized exception.

Under the Katz test, 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.

California v. Ciraolo, 476 U.S. 207 (1986), extends the same public-vantage-point logic this module’s own fence example describes, applying it to the sky rather than the street. Acting on a tip, officers who lacked probable cause for a ground-level search instead chartered a private plane and flew over a suspect’s backyard at 1,000 feet — a height within routinely traveled, publicly navigable airspace — and observed marijuana plants growing in a yard otherwise enclosed by two fences that blocked any view from the ground. The Supreme Court held this wasn’t a “search” at all under the Katz test: a person may have a subjective hope that a fenced yard is private, but that expectation isn’t objectively reasonable when the yard remains plainly visible to anyone flying overhead in lawfully navigable airspace, even though the same view was deliberately blocked from any passerby at street level. The case is a real, useful reminder that the two halves of the Katz test can genuinely pull apart — a person can take real, deliberate steps to shield an area from ordinary observers while still failing to establish a constitutionally protected expectation of privacy against every conceivable vantage point.

Example. An officer looking through a fence into someone’s backyard from a public sidewalk generally isn’t conducting a “search” in the constitutional sense — 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 — more than a hunch, less than probable cause. A Terry stop is a brief investigative detention based on this standard, limited in scope and duration, with any accompanying frisk needing its own separate justification.

Illinois v. Wardlow, 528 U.S. 119 (2000), is the case behind the “unprovoked flight” scenario this section’s own example describes. A man standing in an area known for heavy narcotics trafficking looked in the direction of a passing caravan of police cars and then immediately fled on foot; officers caught up with him, conducted a frisk, and found a handgun. The defendant argued that presence in a high-crime area, combined with a person’s constitutional right to simply walk away from police, couldn’t together add up to reasonable suspicion. The Supreme Court disagreed, holding that unprovoked flight upon noticing police, combined with presence in a high-crime area, is a different matter entirely from a person’s ordinary, protected right to decline an encounter or walk away calmly — flight is the opposite of go-about-your-business behavior, and is the kind of conduct that, together with the location, gives rise to genuine, individualized suspicion under the totality of the circumstances. The Court was careful to note that presence in a high-crime area alone is never enough on its own — it remains just one relevant factor, and requires something more (like the flight itself) to add up to reasonable suspicion.

Example. An officer sees someone matching a witness description near a recent crime scene, who immediately flees when approached — specific, articulable facts that together support a brief investigative stop to ask questions.

Probable cause is a higher standard than reasonable suspicion — facts sufficient to lead a reasonable person to believe a crime occurred — generally required for an arrest or a search warrant, which must be supported by a sworn affidavit and reviewed by a neutral magistrate.

Draper v. United States, 358 U.S. 307 (1959), is a classic illustration of how an informant’s tip, corroborated by an officer’s own observation, can add up to probable cause without a warrant at all. A federal narcotics agent received a tip from a previously reliable informant describing a man who would arrive at a specific train station on one of two given days, wearing particular clothing, carrying a distinctive bag, and walking with a characteristic fast gait — along with the claim that he would be carrying heroin. The agent staked out the station, and on the second day, a man matching every one of those specific, predicted details stepped off the train. The Supreme Court held the agent had probable cause to arrest him without a warrant right there, even though he hadn’t personally observed any criminal activity — the informant’s tip, once so precisely corroborated by the agent’s own observation of predicted, verifiable details, gave the agent reasonable grounds to believe the remaining, unverifiable claim (that the man was carrying drugs) was also true. The case remains a foundational illustration of how officers build probable cause in the field, from a tip plus corroboration, well before Illinois v. Gates later formalized the totality-of-the-circumstances approach to this kind of analysis.

Example. An officer with a sworn witness statement, physical evidence, and a suspect’s own inconsistent story has enough for probable cause to make an arrest — well beyond the reasonable suspicion needed for a brief investigative stop.

Real, well-defined exceptions let officers act without a warrant: plain view, search incident to arrest, the automobile exception, exigent circumstances, and consent — each with its own specific, limited requirements.

California v. Carney, 471 U.S. 386 (1985), tests the automobile exception’s outer boundary — how “vehicle-like” something has to be before its reduced expectation of privacy applies. Agents, acting on a tip, searched a fully mobile motor home parked in a public lot, without a warrant, based on probable cause to believe it contained evidence of a drug transaction. The defendant argued a motor home, used as an actual residence, should be treated like a home requiring a warrant, not like an ordinary car. The Supreme Court disagreed, holding that the automobile exception’s justification — a vehicle’s inherent mobility, plus the reduced expectation of privacy that comes with a vehicle’s pervasive regulation and its use primarily for transportation on public roads — applied here too, since the motor home was readily mobile and located in a place (a public parking lot) not treated as a fixed residence. The Court did leave open that a vehicle rendered genuinely immobile and situated in a way that objectively signals its use as a fixed residence might be treated differently, but the readily mobile motor home here fell squarely within the ordinary automobile exception’s reach.

Example. An officer lawfully inside a home for an unrelated reason spots drug paraphernalia sitting in plain view on a table — no warrant is needed to seize it, since the plain view exception’s three requirements are all satisfied.

There’s no fixed universal time limit on a Terry stop, but it must stay reasonably limited in scope and duration — an unreasonably prolonged or overly intrusive detention risks becoming a de facto arrest, which requires probable cause instead of mere reasonable suspicion.

Florida v. Royer, 460 U.S. 491 (1983), is the case that shows exactly where that line gets crossed. Airport narcotics agents, suspecting a traveler fit a drug courier profile, approached him, asked for and kept his airline ticket and driver’s license, and — without returning either — asked him to accompany them to a small, enclosed room nearby for further questioning, retrieving his luggage from baggage claim without his consent along the way. He eventually consented to a search of his bags, which revealed marijuana. The Supreme Court held that by this point, the encounter had exceeded the bounds of a valid Terry stop and become a de facto arrest requiring probable cause, which the officers didn’t have: moving him to an enclosed room, retaining his identification and ticket, and taking his luggage without permission were all more intrusive than what a brief, on-the-spot investigative stop reasonably requires. The Court emphasized that the government bears the burden of showing an investigative stop was “sufficiently limited in scope and duration” and used the “least intrusive means reasonably available” to confirm or dispel suspicion quickly — and the consent to search, obtained only after this de facto arrest had already occurred without probable cause, was itself tainted by the illegality.

Example. Holding someone for a brief, targeted round of questioning is a valid Terry stop. Holding the same person handcuffed in a locked patrol car for an hour, with no real investigative progress, risks crossing the line into a de facto arrest.

In practice, officers make these judgment calls in real time, under real pressure — department training and policy exist precisely to translate these standards into workable field guidance, since getting the analysis wrong can mean suppressed evidence or a lost case, not just an abstract legal error.

The stakes of getting this analysis wrong run in two real, distinct directions, worth separating clearly. First, a Fourth Amendment violation can trigger the exclusionary rule, suppressing the resulting evidence and potentially unraveling an otherwise strong criminal case entirely — the consequence this module has focused on so far. Second, and separately, an officer (and potentially their department) can face personal civil liability under 42 U.S.C. § 1983 for violating someone’s constitutional rights, regardless of whether any criminal case against that person ever existed at all. That second consequence is generally softened by qualified immunity, which shields an officer from civil liability unless the officer violated a constitutional right that was already clearly established by existing case law at the time — meaning an officer who makes a genuinely novel, good-faith judgment call in an unclear legal area is often protected from a lawsuit even where a court later concludes, in hindsight, that a constitutional violation actually occurred. Both consequences are exactly why clear, specific, well-documented articulation of the actual facts supporting a stop, frisk, or search — not just a correct outcome — matters so much in practice.

Example. An officer who can clearly articulate, in a written report, the specific facts supporting a stop or search gives both the case and their own actions a far stronger foundation than one who can only describe a vague feeling that something was off.