The Midnight Story (1957)

Beloved priest Father Thomasino is murdered in a San Francisco alley, and the police have few clues. But traffic cop Joe Martini becomes obsessed with finding the killer; he suspects Sylvio Malatesta. Ordered off the case, Joe turns in his badge and investigates alone. Soon he is a close friend of the Malatesta family, all delightful people, especially lovely cousin Anna. Uncertain whether Sylvio is guilty or innocent, Joe is now torn between old and new loyalties.