Legendary CIA spy hunter James Angleton erred by returning a turncoat agent to the KGB after mistakenly concluding his cooperation with the United States was a fraud, a new book says.

There are conflicting accounts of what became of the agent, KGB Major Yuri Loginov - that he was shot or merely fired by the Soviet spy agency, according to the book by British journalist Tom Mangold.According to the book, after Loginov had been secretly spying for the CIA from within the KGB for six years, Angleton arranged to have him arrested as a Soviet spy in South Africa in 1967 and swapped against his will two years later across the West-East German border in 1969 for 11 Westerners.

A secret two-year investigation by the CIA concluded in 1979 that Angleton had erred and Loginov had genuinely transferred his allegiance from the KGB to the CIA, writes Mangold, the senior correspondent on the British Broadcasting Corp.'s "Panorama" program.

He says Angleton's error cost the United States a rare opportunity to discover and manipulate an entire network of so-called "illegal" Soviet agents in this country. "Illegals" are usually immigrants in private life.

Mangold's book, "Cold Warrior, James Jesus Angleton, the CIA's Master Spy Hunter," is to be published here and in Great Britain this month. It is the basis for a documentary by Mangold to be shown on Public Broadcasting Service's "Frontline" program Tuesday. An advance copy was made available to The Associated Press.