When the police do not have probable cause to arrest a person suspected of criminal activity—but do have reasonable suspicion the person is involved in criminal activity (a lesser standard than probable cause)—they may stop or detain the person to investigate whether a crime has been committed or is about to be committed—and may frisk or pat down the person to check for weapons.

Such an investigative stop or Terry stop—named after the landmark U.S. Supreme Court case of Terry v. Ohio, 392 U.S. 1 (1968)—is permissible under the Fourth Amendment to the United States Constitution without a warrant because such a person has not been seized within the meaning of the Fourth Amendment. The Supreme Court has suggested that a person is seized within the meaning of the Fourth Amendment when the police officer—by means of physical force or show of authority (siren, flashing lights, display of a weapon, blocking the suspect’s path)—has restrained the liberty of the person. In other words, a stop becomes a seizure only if, in view of all of the circumstances surrounding the incident, a reasonable person would have believed he was not free to leave. And although a police officer may have the right to stop a person based on reasonable suspicion of criminal activity, the stop must be limited in time and scope, or it will become an lawful warrantless seizure, and any evidence gathered by the police during the encounter will not be admissible in evidence against the suspect.

In determining whether there is reasonable suspicion to justify a stop-and-frisk encounter, the test for reasonableness is whether the police officer can identify specific and articulable facts which, taken together with rational inferences from those facts, would lead a reasonable person to conclude that possible criminal activity was in process and that both an investigative stop and a protective frisk or pat down for weapons was required. Because the lawful purpose of a frisk or pat down during an investigative stop is the discovery of dangerous weapons that pose a threat to the police officer, the scope of the frisk or pat down must be limited to finding any such weapons.

A search for weapons during such an investigative stop is not necessarily limited to the body and clothing of the suspect, and may extend to the passenger compartment (seating area, glovebox, console) of a car if the police officer has a reasonable belief, based on specific and articulable facts, that the suspect is dangerous and may gain immediate control of weapons.

And the U.S. Supreme Court has used a reasonableness analysis similar to that for investigative stops of a person when analyzing the reasonableness of an investigative detention of a person’s luggage (but not the person) at the airport—for the purpose of allowing a narcotics detection dog to inspect the luggage. In one case, the Court distinguished such an investigative detention of a traveler’s luggage from an investigative detention of a traveler taken to an interrogation room on grounds of less than probable cause when the police took the traveler’s airplane ticket and retrieved his luggage without permission. Under those circumstances, the Court held, the encounter was not an investigative stop but an unlawful warrantless seizure.

In Michigan, as in other states, the police may conduct a 'Terry stop' or investigative stop if they have reasonable suspicion that a person is involved in criminal activity. This standard is lower than probable cause and allows for a temporary detention to investigate potential crimes. During such a stop, officers may also perform a 'frisk' or pat-down for weapons if they believe the person is armed and dangerous. This is to ensure officer safety. The stop and frisk must be limited in scope and duration to its purpose of confirming or dispelling the officer's suspicion. If the stop extends beyond what is reasonable, it may be considered an unlawful seizure, and any evidence obtained may be inadmissible in court. Additionally, the search for weapons may include areas within the suspect's immediate control, such as the passenger compartment of a vehicle, if there is a reasonable belief that the suspect could access weapons. The U.S. Supreme Court has applied similar reasonableness standards to the investigative detention of luggage at airports, distinguishing between permissible brief detentions for dog sniffs and unlawful seizures when the police exert control over a traveler's belongings without consent or probable cause.