According to original author Max Allan Collins real life gangsters John and Connor Looney's names were changed to Rooney for the film.
The movie is loosely based on actual events and a real enforcer of mobster John Looney who was betrayed.
Maguire is based on Arthur Fellig, nicknamed "Weegee", a photographer who wandered the streets of New York in the 1930s, taking pictures of crime scenes and selling them to the police. The photos in Maguire's apartment are real 1930s crime scene photos, some of which were taken by Weegee himself.