Robert Blake's character is a former Hispanic gang member, which was rare in movies at this time, especially being the only protagonist of the inmates. Also, Robert Blake had played a lot of Spanish characters throughout his career leading to this point, including the little boy who sells Humphrey Bogart the winning lottery ticket in The Treasure of the Sierra Madre (1948). He played Native Americans as well, and later, a "part Cherokee" criminal in his most famous role, In Cold Blood (1967). It wasn't until later that he would be cast primarily as what he actually was, an Italian American.
Decades later, a reunion of this film would occur on a show Robert Blake would become most famous for: Timothy Carey plays a mobster on the pilot of his cop series Baretta (1975).
The "May Day Massacre" the cops read on Lou Gannon's rap sheet was obviously based on The St. Valentine's Day Massacre: The beat cop remembers the bodies piled up in a garage.
Arline Hunter, as "Girl," has no lines, just a scream. She watches as Lou Gannon is killed (in the very beginning and again at the end of the movie). She seems inspired by The Lady in Red in the John Dillinger story, only she doesn't seem to have set up Gannon. She did, however, distract him enough for the cops to move in (on the subway staircase).
Co-stars two actors known for infamous off-screen behavior and extremely intense performances, Timothy Carey and Robert Blake. The cult status of this movie is tied to them. Especially Timothy Carey.