Name of a movie that may be based on the Zodiac Killer?

I can't remember the name of this movie, and I really really want to see it all the way through.

I saw near the end of the movie, and having watched a documentary on the Zodiac Killer a few weeks before I instantly connected the two. In the movie the serial killer wasn't the Zodiac Killer, he had some sort of different name (it's not the 2007 movie). It looked like it was a 70's or 80's movie, maybe early 90's. It may NOT have been based on the Zodiac, but it might have been inspired by the events.

From what I saw, there was a detective and this rather obese guy that the detective had been blaming for the murders. The detective had pretty much stalked the obese guy, becoming obsessed with pinning the crime on him. After one point, the serial killer ties up two teenagers while they're picnicking and stabs them brutally. One of the teenagers was the detective's child (can't remember if it was the male or female). After this the detective goes off the deep end, and his accusations get him fired. The case goes cold and he has no way of proving anything that he's claimed. The detective either loses his job or quits it. After so many years, he forges a letter claiming to be the killer to remind them that the killer is still out there (much like the real Zodiac letters, it claims a body count that is unproven, something like "me - 46 Police - 0".

The detective tracks down the obese man one last time. The obese man, this time, beats the hell out of the detective and ties him up, throwing him in the trunk or back seat. He tells the detective that he was right all along and that the sad part is he'll never be able to prove it, as he murders a final victim with the detective's gun, but keeps his fingerprints off of it. The killer then parks at a public place (a police station I think) and runs in, claiming the detective has just killed the final victim. The detective runs in and murders the killer, only to be murdered himself by police in an attempt to stop him from killing. The movie ends shortly after, I can't remember if the police ever find out who actually committed the murders.

I know this is kind of vague, but I don't want to say something like "it was DEFINITELY a police station that the killer runs into in the final scene" and it actually be somewhere else. Is anyone able to help?