Does a police stop and frisk violate the Fourth Amendment when the initial detention was valid?

However, even though the detention initially was valid, Cairo argues continuing the detention to ask him for identification violated the Fourth Amendment. A detention may last only as long as is necessary to effectuate the purpose of the stop. Royer, 460 U.S. at 500 (plurality opinion). A person "may not be detained even momentarily without reasonable, objective grounds for doing so...." Id. at 498 (plurality opinion); cf. United States v. Thomas, 863 F.2d 622, 628 (9th Cir.1988) ("Each element [of a stop and frisk] ... must be analyzed separately; the reasonableness of each must be independently determined.").

However, after it was established Cairo had not shoplifted, new grounds arose for suspecting him of another crime. Police may continue a detention based on information learned during a detention for one crime, even if the information is relevant to a different crime. Cf. United States v. Medina-Gasca, 739 F.2d 1451, 1453 (9th Cir.1984) (reasonable suspicion may ripen into probable cause to arrest through the occurrence of facts or circumstances after the stop).