I've found Zodiac killer, says former policeman Lyndon Lafferty

Sketches of the 'Zodiac' killer produced by the San Francisco police and witnesses in 1969 (Corbis)

Forty years after he last struck, the Zodiac Killer is alive, aged 91, and enjoying a comfortable retirement in a small city in northern California, it was claimed yesterday.

The notorious murderer, who killed five people in and around San Francisco in the late 1960s, got away with the grisly crimes because of a cover-up by "power-brokers" in the local community, according to a new book.

He embarked on the killing spree after learning that his wife was having an affair with a local judge, according to The Zodiac Killer Cover Up, written by a former police traffic officer called Lyndon Lafferty, who worked on the case in 1970.

Mr Lafferty says that the suspect, identified using the pseudonym George Russell Tucker, came to his attention at the time but efforts to investigate him further were hampered by his police superiors. He has since devoted his free time to the case, aided by a team of six fellow amateur sleuths, which includes former policemen. Their evidence against "Tucker" is outlined over 440 pages. "Official corruption and political intervention forced the investigation into a top-secret, covert status, giving the insane Zodiac killer immunity and a license to kill," they conclude.

The crimes became an international talking point after the killer sent coded letters to local newspapers. Theories about the murderer have spawned multiple books, documentaries and a film directed by David Fincher.

Mr Lafferty says he used a pseudonym to identify his suspect because of concerns about the man's safety.

Police say the crimes remain unsolved. Their only credible suspect, Arthur Leigh Allen, has been cleared by DNA evidence. Dan Olson, an FBI agent in charge of the Zodiac brief, said Mr Lafferty lacks sufficient evidence but could be correct.