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The film's working titles were 29 Clues and The L.A. Investigator. Credits on the viewed print were missing; the above credits were taken from a cutting continuity deposited in copyright records. A written foreword on the viewed print states that this film was based on a case history taken from the detective files of the Los Angeles Police Department, with whose cooperation the film was made. Hollywood Reporter further stated that the film, which had a "semi-documentary" style, was based on the murder of a Pasadena policeman by a youth who worked in the police's fingerprinting department. Daily Variety reports that Los Angeles police aided in research for the film, and that police sergeant Marty Wynn, credited on the screen as technical advisor, was a member of the force. A narrator introduces the audience to Los Angeles through a photographic montage in the film's opening. Actor Jack Webb, who plays a forensics expert in the film, become known for his characterization of Los Angeles police detective "Sgt. Joe Friday" in the popular NBC television series Dragnet, which he also created and narrated, and based on case files of the Los Angeles police. The original series ran intermittently from January 3, 1952 through September 1959 and was revived in a second series, which aired from January 1967 to September 10, 1970. As noted in modern sources, Anthony Mann, who directed Eagle-Lion's early 1948 semi-documentary release T-Men (see below), completed the directing assignment on He Walked By Night for Alfred Werker. Several reviews lauded the film's final chase scene, shot in what New York Times called "700 miles of hidden highways" that make up the giant underground tunnels of Los Angeles' storm drain system.