The controversial defection of the Soviet agent Yuri Nosenko to the United States contributed to deep and demoralising divisions within the CIA and the ultimate dismissal of James Jesus Angleton, the agency’s chief of counter-intelligence.

Nosenko joined the KGB’s Second Chief Directorate in 1953 and was responsible for the surveillance and recruitment of foreigners in Moscow. In the 1960s he offered to provide the West with details of Soviet penetration agents and, later, information about the KGB connection with Lee Harvey Oswald, the assassin of President Kennedy.

His defection was initially viewed as a CIA coup, but it came at a time when the agency was gripped with spy fever, stoked by Angleton, and fears that the CIA had been penetrated by a Soviet mole in its highest ranks.

Angleton had fallen under the spell of another KGB defector, Anatoli Golitsin, who had warned him of the presence of a high-level mole. Golitsin predicted that Moscow would sacrifice a false defector to deflect counter-intelligence investigations and try to discredit him. Despite the fact that Nosenko had provided valuable information about Soviet espionage, Angleton became convinced that he was the disinformation agent sent by Moscow.

Nosenko was accepted by the Americans as a defector largely because of his knowledge of the KGB’s relations with Oswald, which was thought to be crucial to the Warren Commission’s investigation into the Kennedy assassination. Nosenko had interviewed Oswald during his time in the Soviet Union and, after the murder had reviewed the KGB’s Oswald file.

Cross-examined about Oswald after his defection, Nosenko asserted that the KGB had determined that Oswald was too unstable to be taken on as an agent and maintained that the KGB had not recruited him to kill Kennedy. Such categorical assurances of Russian innocence, coming so soon after the assassination, seemed too convenient and fired Angleton’s suspicions that Nosenko was a KGB plant.

In April 1964, two months after his defection, Nosenko was imprisoned at Camp Perry, a CIA facility in Virginia, and illegally held for three years in solitary confinement under harsh detention with endless hostile interrogations and multiple polygraphs. In a deposition to Congress later, he said he was allowed to shower and shave only once a week, had no toothbrush or toothpaste, no one to talk to or books to read and he was always hungry. A reporter for The New York Times, Tim Weiner, in his book Legacy of Ashes, recounted that Angleton

“believed that if the CIA could break Nosenko, the master plot might be revealed — and the Kennedy assassination solved”.

After numerous lie-detector tests and interrogations the CIA determined that Nosenko, despite contradictions in his testimony, was on the whole telling the truth. He was freed in 1967 with $80,000 and a new name, and sent to spend the rest of his life in the South, with occasional trips to the CIA headquarters to lecture American intelligence professionals.

Angleton and Nosenko’s main CIA handler, Tennent Bagley, continued to insist that Nosenko was a KGB plant but had diminishing support for this view within the agency. Clair George, a former CIA deputy director of operations, said on Nosenko’s death that his treatment had been “a terrible mistake”. But, “you can’t be in the spy business without making mistakes”.

The Nosenko affair did nothing to undermine Angleton’s confidence in the defector Golitsin. However, his failure to prove his case against Nosenko, which became a cause célèbre in the US intelligence community, did much to undermine his authority.

Angleton’s growing paranoia over Soviet penetration of the CIA demoralised and paralysed the agency for years. Espionage operations, according to one senior officer of the CIA’s Soviet bloc division, were “dead in the water”, intelligence careers were ruined, operations paralysed and relations with friendly intelligence services crippled. So much damage was done to the agency by Angleton’s witch-hunts that some professionals began to suspect that Angleton himself must have been a KGB plant. Others date his paranoia from his being duped by his friend and mentor, the British spy Kim Philby. Angleton and Philby used to meet regularly in Washington in the 1950s for Martini lunches right up until Philby’s unmasking. It was said that from the time Philby was denounced as “the third man” Angleton trusted no one.

Nosenko, codenamed Foxtrot by the CIA, was born in 1927 in Nikolaev, in Ukraine, and was educated by private tutors. He served for three years in naval intelligence after his 1950 graduation from the State Institute of International Relations in Moscow and then joined the KGB internal security division. He developed a fondness for Western culture, drink and womanising and made his first contact with US Intelligence in Geneva in 1962 when a member of the Soviet Union disarmament delegation. He offered to supply information, pleading for money to replace KGB funds which he had squandered on alcohol and a prostitute at a local nightclub.

At that time he made some significant intelligence disclosures. One was that a homosexual British Embassy official, later identified as the Admiralty clerk John Vassall, had been caught in a Moscow honeytrap and blackmailed to work for the KGB. He also provided information that the walls of the American Embassy in Moscow had been embedded with hundreds of tiny listening devices and that a homosexual Canadian Ambassador in Moscow had been compromised by the KGB. All charges proved accurate. He also revealed that British intelligence had been penetrated by Moscow at a high level.

After this approach to the CIA, Nosenko returned to Moscow. Two years later he was back in Geneva with the disarmament delegation and defected to the CIA, claiming that the KGB was on to him. He was put into an American army officer’s uniform and whisked across the border to a safe house in Frankfurt where he was debriefed. In the two months before his three-year incarceration he proved difficult to handle, getting involved with drink and bar brawls over women.

Despite his long confinement he never complained. “While I regret my three years of incarceration, I have no bitterness and now understand how it could have happened,” he said later.

One month before he died and after 41 years of living under an assumed name at a secret address, several senior CIA officials visited Nosenko to present him with the Stars and Stripes and a letter from Michael Hayden, the director of the CIA, thanking him for his service and, by implication, offering him a final apology.

Yuri Nosenko, spy, was born on October 30, 1927. He died on August 23, 2008, aged 80