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 Texas, as in other states, the principles established by the U.S. Supreme Court in Terry v. Ohio allow police officers to conduct a brief, investigatory stop of a person if they have reasonable suspicion that the person is involved in criminal activity. This is a lesser standard than probable cause, which is required for an arrest. During such a Terry stop, officers may also perform a frisk or pat down for weapons if they have a reasonable belief that the person may be armed and dangerous. The stop and frisk must be limited in scope and duration to that which is necessary to confirm or dispel the officers' suspicions. If the stop extends beyond what is reasonable, it may be considered an unlawful seizure, and any evidence obtained may be inadmissible in court. In Texas, as per federal law, the scope of a frisk can extend to 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 a weapon. These principles are grounded in the Fourth Amendment's protection against unreasonable searches and seizures and aim to balance individual rights with officers' need to investigate potential criminal activity and ensure their own safety.