From Wikipedia:
He Walked by Night (1948) is a black-and-white police procedural with film noir styling, crediting Alfred L. Werker as director. In reality, most of the film was directed by western/film noir director Anthony Mann. The film, shot in semidocumentary tone, was based on the real-life actions of Erwin "Machine-Gun" Walker, a psychopathic killer whose story and capture inspired one of the film's actors (Jack Webb) to create the radio and later television program Dragnet.