In what circumstances will a police officer be permitted to frisk and search a group of people who they suspect may be about to commit a crime?

In Terry v. Ohio (1968) 392 U.S. 1, 88 S.Ct. 1868, 20 L.Ed. 889, the court sustained a police officer's accost and frisk of loiterers on the street who were behaving suspiciously and were suspected by the officer of planning a holdup. The court recognized 'that a police officer may in appropriate circumstances and in an appropriate manner approach a person for purposes of investigating possible criminal behavior even though [30 Cal.App.3d 262] there is no probable cause to make an arrest.' (392 U.S. at p. 22, 88 S.Ct. at p. 1880.) Explicit in the decision is approval of police investigation 'where a police officer observes unusual conduct which leads him reasonably to conclude in light of his experience that criminal activity may be afoot . . .' (392 U.S. at p. 30, 88 S.Ct. at p. 1884.)