The film does not show that Carnes, after escaping from the Granite Reformatory in Oklahoma, carjacked a passing driver and forced the driver to take him across the state line. That resulted in federal charges of kidnapping and crossing state lines to flee prosecution. That was the reason he was sent into the federal prison system; which resulted in him being sent first to United States Penitentiary, Leavenworth, and then to Alcatraz.
Michael Beck spent one night confined by himself in a prison cell to research his role as Clarence Carnes.
The real Clarence Carnes worked as a consultant on this movie.
Telly Savalas (Joe Cretzer) appeared in Birdman of Alcatraz (1962), starring Burt Lancaster as Robert Stroud. In the previous film, Savalas played a convict, Feto Gomez, at Leavenworth prison, and, like Stroud, was later sent to Alcatraz.