Martin Balsam's character, Mac Keeley, was based on a real-life Chicago Tribune reporter named Jake Lingle. Lingle, a "legman" who ran down gang-related stories for the paper, had close ties to Al Capone and other gangsters as well as the notoriously corrupt Chicago Police Department, and he was well-paid by both mobsters and a police commissioner as a "go-between." Lingle was gunned down on June 9, 1930, much as depicted in the movie, after "getting too big for his hat", as Capone put it, and demanding too much for his services (though a Capone rival likely paid for the hit). Apparently legal concerns prevented the producers of this film from using Lingle's name. However, just a few months after this film was released, the TV series Os Intocáveis (1959) told Lingle's story in its third episode and used his actual name.
Al Capone's sister sued the filmmakers for $10M for invasion of privacy and for failing to get permission to make this film. The suit was dismissed in 1962.
The real Al Capone died of advanced syphilis which had become neurosyphilis. Due to the production code in effect at the time, the narrator (James Gregory) attributes Capone's death to an "incurable disease".
Like several other films in which Al Capone is depicted, this film makes no reference at all to Capone's wife or children.
The film's (fictitious) sub-plot in which Capone successfully woos the widow of a man whose death he has arranged (as she knows) is borrowed from Shakespeare's "Richard III".