A KGB spy who switched allegiances at the height of the Cold War and was considered by the CIA as its “most valuable and economical defector” has died.

Yuri Ivanovich Nosenko defected in Switzerland in 1964. Confined to a safe house in Clinton, Md., the former Soviet spy was interrogated for about four months in 1965 until transferred to a specially constructed jail because he was suspected of being a double agent, according to decades-old CIA documents released last year. He was held until October 1967, then resettled under an assumed identity.

“While I regret my three years of incarceration, I have no bitterness and now understand how it could happen,” he said, according to the documents.

The 81-year-old Nosenko died Saturday, a month after the CIA delivered to his home a ceremonial flag and a letter of thanks from the agency’s director, Michael Hayden, honoring his service to the United States, according to intelligence officials.

The CIA considered him the “most valuable and economical defector this agency has ever had,” the long-held documents said, noting his information resulted in the arrest and prosecution of spies.

The CIA put Nosenko under a polygraph in 1964, 1966 and 1968 about Kennedy assassin Lee Harvey Oswald’s possible ties to the KGB. Nosenko told his interrogators that Oswald was not a KGB operative, according to a 1979 report to Congress.

Nosenko’s death after a long illness was first reported in Wednesday’s Washington Post, which said he lived in a Southern state.