Since the 1968 decision in Terry v. Ohio, the U.S. Supreme Court has divided police infringements on a suspect's liberty into two principal categories: detentions and arrests.
A detention occurs when an officer has said or done something that would cause a reasonable innocent person to believe he is not free to disregard the police presence and go about his business. (Florida v. Bostick) A detention must be justified by "reasonable suspicion" of criminal activity on the part of the detainee. Under this standard, it is counter-productive error for officers to speak of "PC for the stop." Probable cause is never constitutionally required for detentions. (US v. Sokolow)
An arrest occurs when a person is told he is under arrest and submits to custody, or when the person is restrained beyond the bounds of a temporary detention (such as being involuntarily transported to the police station for investigation). (Kaupp v. Texas) An arrest must be supported by "probable cause" to suspect the arrestee of a criminal act. (Beck v. Ohio) Although it isn't always easy to determine when an encounter that starts as a detention ripens into an arrest, some of the characteristics of each level can be set apart.
Detentions include the pedestrian stop, the vehicle stop, and the restraint of occupants while a search warrant is being served. The lawfulness of this latter kind of detention is rarely in question, because the Supreme Court has ruled that the existence of a valid search warrant "implicitly carries with it the limited authority to detain the occupants of the premises while a proper search is conducted." (Michigan v. Summers) The lawfulness of ped stops and vehicle stops, on the other hand, has been the subject of numerous decisions attempting to define the circumstances that may justify the detention, and to set the boundaries of permissible investigative activity.
What Does "Reasonable Suspicion" Mean?
Reasonable suspicion is "a particularized and objective basis for suspecting the particular person stopped of criminal activity." (US v. Cortez) The facts known to the officer and the inferences drawn based on the officer's training and experience "need not rule out the possibility of innocent conduct." (US v. Arvizu) Reasonable suspicion is a lower level of justification than the probable cause required for arrest, and it can be established with evidence that is lower in both reliability and amount than would be needed for PC, as the court has said in a pair of decisions:
"The police can stop and briefly detain a person for investigative purposes if the officer has a reasonable suspicion supported by articulable facts that criminal activity may be afoot, even if the officer lacks probable cause. The officer, of course, must be able to articulate something more than an inchoate and unparticularized suspicion or hunch.
"The level of suspicion required for a detention is obviously less demanding than that for probable cause. The Fourth Amendment requires some minimal level of objective justification for making the stop. That level of suspicion is considerably less than proof of wrongdoing by a preponderance of the evidence." (US v. Sokolow)
"Reasonable suspicion is a less demanding standard than probable cause not only in the sense that reasonable suspicion can be established with information that is different in quantity or content than that required to establish probable cause, but also in the sense that reasonable suspicion can arise from information that is less reliable than that required to show probable cause." (Alabama v. White)