A law enforcement officer can perform a temporary investigative detention of an individual only when the officer can articulate specific facts, which considered in light of the circumstances, provides an objective reason to believe the individual is engaged, or about to engage, in criminal activity. (A detention occurs when an individual does not feel that he or she is free to leave, as contrasted with a consensual police encounter in which a person can just walk away.)
Note that the officer’s reasons for believing a detention is warranted is an objective standard—the officer’s assessment must be such that a law enforcement officer presented with the same circumstances would reasonably suspect possible criminal activity. Thus, an officer who detained an individual on the suspicion that the individual was engaged in criminal activity simply because that individual is walking in a high crime area would not be objectively reasonable. But if that individual is also observed by the officer in what appears to be a drug transaction, a detention might be objectively reasonable.
Lawful detentions have often been a bone of contention between defense attorneys and the prosecution, and the courts have often taken a fairly hard line on the subject. For example, the courts have held that a lawful detention can be found simply on an individual’s attire, demeanor, evasiveness and other ambiguous circumstances. Whether a detention is lawful or not is important because a lawful detention often leads to a search of and if the officer discovers a crime (for example, find drugs or a weapon on the individual), any subsequent charges can be dismissed if the defendant’s criminal defense attorney files a successful motion to suppress evidence on grounds that the detention was unlawful.