The Laughing Policeman (1973)

Synopsis
Stuart Rosenberg's taut police thriller, based on the Martin Beck novel by Maj Sjowall and Per Wahlöö, concerns a serial killer who is targeting bus riders. When a bus full of innocent commuters are killed on a San Francisco street, Jake Martin (Walter Matthau) is assigned to track down the killer. Jake has a personal stake in the killings because his partner was one of the victims. Teamed up with new partner Leo Larsen (Bruce Dern), Jake investigates the back alleys of San Francisco to find the serial killer. The trail leads to a tour of the underbelly of the city's gay subculture. ~ Paul Brenner, All Movie Guide