The story based the film's four detectives on the real life LAPD foursome known as the Hat Squad. As portrayed in the film, the LAPD leadership empowered them and the larger "Gangster Squad", subsequently renamed the "Intelligence Division", to specifically interdict the movement of East Coast organized crime into Los Angeles. The actual LAPD units allegedly used tactics commensurate with the level necessary to counter and repulse organized crime groups.
Jack Nicholson was offered the role of Chief Bill Parker. He declined, but he did recommend Bruce Dern for the part.
The car driven by The Hats is a 1949 Buick Roadmaster convertible, with its famous four round "Ventiports" on its front fenders.
About nudity in the movie, Jennifer Connelly said: "It kind of shocked everyone who knows me that I wound up doing this movie, because I had always been so careful about nudity, it was very much a part of this character and I couldn't be coy or guarded or self-conscious, otherwise it wouldn't work. It was sort of a challenge I wanted to take on, I guess."
Upon its release, the film was somewhat compared to Chinatown by some critics. The look of the film was by production designer Richard Sylbert, who was the production designer on that film, the time period (1940's), the setting (Los Angeles), and the films original score by Dave Grusin, which features a bit of Jerry Goldsmith's stylistic qualities while keeping his own musical voice.