A killer who gave himself the nickname "Zodiac" is believed responsible for series of murders in Northern California during the 1960's and '70's; however, police could never find enough evidence to identify the suspect and, officially, the cases remain unsolved. One man's obsession to unravel the mystery led to a best-selling book which is now adapted into the latest film by director David Fincher. Alan Silverman has a look at Zodiac.

The murder spree begins in 1968 with several seemingly unrelated killings in suburban communities around San Francisco Bay. What ties them together are letters, apparently written by the killer, sent to San Francisco newspapers with demands that they be published or he will kill again.

Accompanying the letters are ciphers - coded messages - that the killer says contain clues to his identity. Robert Graysmith, a political cartoonist at one of the papers (the San Francisco Chronicle) and an amateur cryptologist becomes fascinated ...no, obsessed ...with solving the case.

The film is based on Graysmith's books in which he deduces the Zodiac killer's identity, but the circumstantial evidence never gave police enough facts to make a solid case. On top of that, the author believes that the killer purposely chose to commit the crimes in different jurisdictions because he knew that back then police rarely shared clues with their counterparts in neighboring communities.

"So it took 10 years. I took 10 years to write it," he explains. "You would go to the police station and they would not share the information with anyone, not even with San Francisco or Napa or any of those people. Just for (the) Lake Herman (murders) you had four different departments ...plus a horde of reporters, all of which took all that information home and never shared it. So basically what I did was went and talked to each of them, consolidated it and just kept building from that center. If today they were doing that, they would have a task force. It would be over. They would simply consolidate their information and run it through a computer; and, of course, they have all this new technology.

Mark Ruffalo co-stars in the film as the lead San Francisco homicide detective on the case and says his best source for research proved to be the actual person.

"I spent a lot of time with Dave Toschi, the guy I was playing, and he knew the dates, where he was, what he was wearing ...such beautiful recall, even now ...and he really hasn't talked about this case in years and years," Ruffalo says.

Ruffalo adds that the film's director, who grew up in the San Francisco Bay area at the time of the Zodiac murders, also shared his near-encyclopedic knowledge of the cases with the cast.

"David Fincher is equally obsessed with this and was able to, because of who he is and because we were making a movie, get information that most of the police who worked on the investigation couldn't get from so many different sources," he says. "So we had all of that at our fingertips. I was also privy to the real press conferences, interviews, the murder book ...I had every report on every investigation, every murder that they thought may have been connected to the serial killer, all the letters, the pictures ...I had everything."

Jake Gyllenhaal plays Robert Graysmith and says he came to understand the obsession with the specific cases and the general topic.

"What's great about those stories is that we all have inside of us those ideas - that weird, unconscious idea," Gyllenhaal says. "The idea of a serial killer fascinates us and you can experience it in your everyday life."

It cost Graysmith his marriage, but the author admits that, like the police detectives involved, he became focused on solving the crimes.

"You don't realize it at the time. You just get swept away and, believe me, there are still people who get swept away," he notes. "I get people calling me all the time who begin by saying '...if you knew anything about Zodiac ...' It's too tantalizing. There's never been a case that you can puzzle over and has so much."

Robert Graysmith did track down the man he believes committed the murders and did, for a brief moment, look him in the eye.

Zodiac, directed by David Fincher, also features Robert Downey Jr., Anthony Edwards and Brian Cox. Several scenes were filmed in the actual locations where the murders occurred more than 30 years ago.