The Supreme Court has placed police-citizen encounters into three tiers or categories:
First, there are communications between officers and citizens that are consensual and involve no coercion or restraint of liberty. Such encounters are outside the scope of the Fourth Amendment. A person is not seized within the meaning of the Fourth Amendment, however, merely because a law-enforcement officer approaches and asks a few questions. A seizure occurs only when, taking into account all the circumstances, the law-enforcement officers’ conduct would have communicated to a reasonable person that he or she was not free to ignore the police presence and go about his or her business.
Second, there are the so-called Terry-type stops. These are brief, minimally intrusive seizures but which are considered significant enough to invoke Fourth Amendment safeguards and thus must be supported by a reasonable suspicion of criminal activity. During a second-tier encounter, the police officer actually conducts a brief investigative stop of the citizen. In this level, a police officer, even in the absence of probable cause, may stop persons and detain them briefly, when the officer has a particularized and objective basis for suspecting the persons are involved in criminal activity. To stop a citizen, the officer must possess more than a subjective suspicion or hunch. The officer’s action must be justified by specific facts which, taken together with rational inferences from those facts, reasonably warrants that intrusion, and the detention cannot be either arbitrary nor harassing. In short, an investigatory stop must be justified by some objective manifestation that the person stopped is, or is about to be, engaged in criminal activity.
Third, there are highly intrusive, full-scale arrests, which must be based on probable cause. Probable cause exists when the arresting officer has knowledge or reasonably trustworthy information sufficient to warrant a person of reasonable caution in the belief that an offense has been committed by the person to be arrested.