The film was inspired by real life cases. The first was Richard Speck's murders of eight student nurses. The second inspiration was when a Scotland Yard investigator got fired from his job for planting evidence to convict a Thames River killer, which turned out that the killer committed three murders before being convicted. And the killer is also is quite similar to Ted Bundy, being that he killed young beautiful women, he was good looking, and he drove a Volkswagon Bug.
This is the first film that Charles Bronson started with the Cannon Group but not Golan/Globus Productions
(At 1:33:07) Warren Stacy picks up the police radio and it says, "Code 6 Florence and Normandie". Nine years later, the Los Angeles Riots (AKA the Rodney King Riots) Would break out starting at that Intersection.