The street with no name (1948)

After two gang-related killings in "Center City," a suspect (who was framed) is arrested, released on bail...and murdered. Inspector Briggs of the FBI recruits a young agent, Gene Cordell, to go undercover in the shadowy Skid Row area (alias George Manly) as a potential victim of the same racket. Soon, Gene meets Alec Stiles, neurotic mastermind who's "building an organization along scientific lines." Stiles recruits Cordell, whose job becomes a lot more dangerous.