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".
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 The Untouchables (1959) told Lingle's story in its third episode and used his actual name.