The Mob (1951)

After witnessing what appears to be a police officer shooting a man in cold blood, detective Johnny Damico (Broderick Crawford) decides to go undercover. Tracing the murder victim to a waterfront crime ring, Damico poses as longshoreman Tim Flynn, and infiltrates the corrupt New York dockworkers union. There he meets union toughs Joe Castro (Ernest Borgnine) and Gunner (Neville Brand), who eventually lead him to crime kingpin Blackie Clegg (Matt Crowley).