Special Investigator [1936] [DVD]

From imdb: Bill Fenwick, criminal lawyer, is becoming a mouthpiece for mobsters, when his brother George, a Justice Department investigator, is killed by mob chief Edward Selton. In atonement, Bill leaves his practice and his venal girlfriend and goes undercover in Nevada to investigate a scheme for "laundering" stolen gold bullion. He starts making progress when he pursues attractive Virginia, who's somehow involved with the gang; in fact, unknown to Bill, she's Selton's sister.