The Devil's Henchman(1949)

Insurance agent Jesse Arno is posing as a sailor while on the trail of a gang of waterfront thieves, supposedly headed by Tip Banning. Arno is aware that a gang member has been murdered by Rhino, Banning's simple-minded right-hand man, but says nothing when he is questioned by the police, who are unaware of his real profession. Banning, knowing that Jesse knew who the killer was, admits him into the gang.