Trivia

James Whitmore debuted in this film in Chicago, Illinois, and on television on the same day - March 20, 1949 - in Dinner at Antoine's (1949) starring Steve Cochran, also in his television debut. Whitmore's next movie role, Battleground (1949), earned him an Oscar nomination.

Based on the Internal Revenue Service Criminal Investigators who took down Al Capone after the Police and FBI were unable to make anything stick because of his ability to pay off or intimidate any witnesses that came forward - just like in this movie. The character of Frank Warren (Glenn Ford) represents real-life agent Frank J. Wilson, the lead agent who finally took down Capone, and the author of the autobiography "Undercover Man" that was serialized in Collier's magazine in 1947 and is the basis for this film.

While the film ostensibly takes place in Capone's Chicago (two decades after the fact), just exactly where its story takes place is never made explicit. Plenty of obvious (money saving) Los Angeles exteriors are used, the cars carry California license plates, and the young runaways frolic on the beach in Malibu. The tenement district and street vendors scenes were evidently shot on the studio's "New York City" back lot. The only actual clue given is that Warren's wife goes to stay at her folks' farm "a couple of hundred miles away" in "Tower City: Dairyland USA", potentially putting it to the north of Chicago in some of America's finest dairying.

Leo Penn, who played accountant Sydney Gordon, was the father of actors Sean Penn and Chris Penn.

Frank and George are shown arriving at an airport on an United Airlines Douglas DC-4, registration N30062, originally made in 1944 as a C-54 transport for the U.S. Army Air Force. In 1955, this aircraft flying as United Flight 409 from Denver, Colorado to Salt Lake City, Utah, crashed near Medicine Bow Peak, Wyoming with the loss of all 66 aboard.