The U.S. Supreme Court allowed stop-and-frisk searches in Terry v. Ohio (1968). A police officer need only have a reasonable suspicion of criminal behavior to detain and question a person (the "stop"). An officer who has reason to believe the detainee may be armed and dangerous can at the same time carry out a limited pat-down search for weapons (the "frisk"). This rule applies whether the suspects are on foot or in a car. A "reasonable suspicion" requires more than a hunch or a mere distrust; the officer must have objective grounds, based on all of the circumstances, to suspect that a person is involved in criminal activity.
Example: Officer Crosby sees Stills and Nash talking normally on a street corner. Having a hunch that a drug transaction may be underway, Crosby detains and frisks the pair. He finds a gun in Nash’s pocket and arrests him. Under these circumstances, Officer Crosby had no right to detain Stills and Nash in the first place. A "hunch" doesn’t authorize detention; an officer must have "articulable facts supporting a reasonable suspicion." (U.S. v. Hensley, U.S. Sup. Ct. 1985). Because the initial detention was improper, the frisk incident to that detention was also improper, and the result of the frisk—the gun—is inadmissible.
ILLEGAL DETENTION DOESN'T ALWAYS MEAN INADMISSIBLE EVIDENCE
The exclusionary rule generally requires that evidence resulting from an unlawful detention or arrest be excluded from court. But not always.
Suppose a warrant is out for your arrest. An officer who’s unaware of the warrant detains you, but not because you were doing anything wrong. Maybe the officer is speculating that you’re up to no good. Whatever the reason, it’s not reason enough: The policeperson doesn’t have reasonable suspicion that you have committed or are committing a crime.
The officer asks for your identification, has dispatch run a check, and learns about the warrant. The cop arrests and searches you, finding some kind of contraband in your pockets.
Even though the stop was illegal because it wasn’t based on reasonable suspicion, the contraband could very well be admissible against you in court. The U.S. Supreme Court has held, essentially, that arrest warrants can retroactively justify illegal detentions. (For much more on the rules in this area, see this article on the “attenuation doctrine.”)
Example: Officer Jacks sees Jill hiding under the steps of an apartment building. As the officer approaches, Jill runs away while grasping for something in her pocket. Officer Jacks chases and detains Jill and pats her down for weapons. The officer removes a hard object that turns out to be a plastic envelope containing burglar’s tools. Officer Jacks can legally seize the tools because Jacks had a reasonable basis for suspecting that Jill was engaged in criminal activity, and that she might have had a weapon. The officer had the right to detain Jill, pat her down, and remove an object that might have been a weapon.
Example: Officer Ross spots Wade’s minivan on a little-used road sometimes frequented by drug smugglers. Wade is driving at a time when border patrol officers commonly change shifts. Officer Ross, who knows that drug smugglers often use minivans, runs a check on the vehicle and finds that it is registered to an address in a block notorious for drug smuggling. Officer Ross stops Wade and asks to search the van, and Wade consents. A subsequent search of the minivan reveals 130 pounds of marijuana. Under these circumstances, Officer Ross had a reasonable suspicion that Wade was engaged in illegal behavior. Because the stop was legal and the resulting search was consensual, the marijuana is admissible as evidence. (U.S. v. Arvizu, U.S. Sup. Ct. 2002).