There has been no shortage of theories about the identity of Zodiac killer, who struck Northern California four decades ago.

And interest has continued even as police quietly closed the books on the killings year ago.

Now, there is a new scenario making the rounds.

"The Zodiac Killer Cover Up" is a new book written by a former California Highway Patrol officer, Lyndon Lafferty.

Lafferty claims the killer is now a 91-year-old man living in Solano County. The book uses aliases and does not identify the alleged killer by name. Lafferty claims he and other lawmen investigated the suspect in the early 1970s but were stymied by "power brokers" in Solano County.

"Look, I know what most people think of Zodiac theories, and there have been so many over the years that if a person walked into the Vallejo Police Department right now and said he was the Zodiac, they'd tell him to get out," Lafferty told the San Francisco Chronicle in an interview last year. "But I do feel that we have a very credible case."

Lafferty claims the killer was motivated by jealousy because his wife was having an affair.

The Zodiac killer is blamed for at least five slayings in 1968 and 1969. There was never an arrest in the case.

Three killings occurred in Vallejo. Teenagers David Farraday and Betty Lou Jensen were shot to death in December 1968. Darlene Ferrin, 22, was shot and killed seven months later at Blue Rock Springs Golf Club. Her companion, Michael Mageau, 19, survived.