Investigatory detentions: if police have reasonable suspicion of criminal activity or involvement in a completed crime, supported by articulable facts, they may detain a person for investigative purposes; if police also have reasonable suspicion that detainee is armed and dangerous, they may frisk for weapons; detention must be no longer than necessary to conduct a limited investigation to verify suspicion; if during detention probable cause arises, the detention becomes an arrest; brief property seizures are similarly valid if based on reasonable suspicion