An officer is allowed to conduct a stop and frisk (sometimes called a “Terry Stop” after the Supreme Court case that established their legality, Terry v. Ohio) when it is reasonable to believe that criminal activity may be taking place or is about to take place. The Court authorized the practice with the intention of improving police officer safety. Many crimes involve the use of dangerous weapons, and so, the rationale goes, it is reasonable to search for weapons when a person may be committing a crime.
Because a stop and frisk is supposed to be a search for dangerous weapons, an officer is not supposed to investigate further into objects which are obviously not weapons. A soft, powdery substance encased in a plastic bag, for example, could not be investigated further because the officer does not have reason to believe that the object could be a threat to officer safety.
The problem comes with how nebulous an officer’s suspicions can be. The Supreme Court has specifically said the standard for whether an officer can conduct a stop and frisk is lower than probable cause (which is low to begin with). The even lower standard is a “reasonable articulable suspicion” that a crime is taking place or is about to take place.
In plain language, officers must be able to explain to judges why they believe a specific person was about to commit a crime or was in the process of committing a crime. However, the Supreme Court has not required a particularly high threshold for what amounts to reasonable suspicion. In Illinois v. Wardlow , the Court held that running away from an officer in a “high crime area” was enough to arouse reasonable suspicion. With such a low threshold, judges often find in the officer’s favor.