The plot is based on the murders committed by the Zodiac Killer in the San Francisco area, though it takes many liberties with the actual investigation with the film providing a name and back story for the killer.
The Zodiac Killer: Directed by Tom Hanson. With Hal Reed, Bob Jones, Ray Lynch, Tom Pittman. A serial killer in San Francisco murders without motive or apparent pattern and taunts the police with phone calls and letters.