A man walking down the street appears to have an illegal weapon. What can a police officer do?

The answer

If a person appears to be carrying an illegal weapon, an officer with reasonable suspicion may lawfully stop the person and frisk them — pat down the outer layer of clothing to check for weapons. This limited action is called a stop and frisk, or a Terry stop, after the Supreme Court case Terry v. Ohio (1968).

A frisk is not a full search. It is confined to a pat-down of the outer clothing for the officer's safety, to find weapons. To go further — reach into pockets, search a bag thoroughly, or make an arrest — the officer needs the higher standard of probable cause.

Reasonable suspicion vs. probable cause

The whole question turns on which legal standard the facts meet. These are two rungs on a ladder under the Fourth Amendment, which protects against unreasonable searches and seizures:

• Reasonable suspicion is a lower bar. It requires specific, articulable facts — not just a hunch — that criminal activity may be afoot and that the person may be armed and dangerous. This authorizes a brief investigative stop and a protective frisk.
• Probable cause is a higher bar. It requires enough facts for a reasonable officer to believe a crime has been (or is being) committed by this person. This is what's needed to arrest, or to conduct a full search (or to get a warrant).

So 'appears to have an illegal weapon' typically supplies reasonable suspicion — enough to stop and frisk. If the frisk reveals an obvious weapon or contraband, that can ripen into probable cause, justifying arrest and a fuller search.

Ruling out the wrong answers

• 'The officer must get a warrant first.' Wrong here — Terry created a recognized exception. A brief, on-the-street protective stop and frisk based on reasonable suspicion does not require a warrant, precisely because the danger is immediate.
• 'The officer can immediately arrest him.' Wrong — merely appearing to have a weapon does not, by itself, establish probable cause for an arrest. The officer may detain and frisk, not arrest, until facts rise to probable cause.
• 'The officer can do a full search of his pockets and bag.' Wrong — a frisk is limited to a pat-down of outer clothing for weapons. A full search requires probable cause or consent.
• 'The officer can do nothing without the man's consent.' Wrong — that ignores Terry. Reasonable suspicion authorizes the stop and frisk without consent.

The bigger picture

Terry v. Ohio balanced two things: an individual's Fourth Amendment right to be left alone, and an officer's legitimate need to protect themselves during a brief investigation. The Court held that the balance permits a narrow intrusion — a protective pat-down — on less than probable cause, so long as the officer can point to concrete facts. The escalation ladder is the key idea to memorize: a mere hunch justifies nothing; reasonable suspicion justifies a stop and frisk; probable cause justifies a search and arrest; and a warrant (or a recognized exception) is generally required for searches of homes and private spaces.