Gangs of New York (1938)

The undercover cop Rocky Thorpe infiltrates a crime syndicate being run by the incarcerated mob boss John Franklin. Franklin conducts his business via a short-wave radio concealed in his cell. One day Franklin is caught and placed in solitary confinement. Thorpe, Franklin's physical double, takes his place. Soon changes in the gang's activities are subtly made. Thorpe orders the mob to keep careful records of their activities, to gather enough evidence to convict them all. Trouble ensues when Franklin escapes from jail.