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 New Mexico, as in other states, the standards for police conduct during encounters with individuals are guided by federal constitutional principles, particularly the Fourth Amendment. The concept of a 'Terry stop,' derived from the U.S. Supreme Court case Terry v. Ohio, allows police officers to stop and detain a person for investigation based on reasonable suspicion of criminal activity, which is a lesser standard than probable cause. During such a stop, if the officer has reasonable suspicion that the person is armed and dangerous, they may perform a frisk or pat down to check for weapons. This is to ensure officer safety and must be limited to this purpose. If the stop or frisk extends beyond the scope of ensuring safety or determining the presence of weapons, or if it becomes too prolonged without evolving into an arrest based on probable cause, it may be considered an unlawful seizure, potentially rendering any evidence obtained inadmissible in court. The reasonableness of a stop is determined by whether specific and articulable facts exist that would lead a reasonable person to suspect criminal activity. In New Mexico, as elsewhere, these principles also apply to the search of a vehicle's passenger compartment under similar circumstances of reasonable suspicion. The reasonableness standard is also used in situations involving the investigative detention of luggage at airports, distinguishing between permissible brief detentions for inspections and unlawful seizures that infringe on a person's Fourth Amendment rights.