WASHINGTON — Yuri Nosenko, a former Soviet agent who was at the center of some of the most dramatic espionage episodes of the Cold War, died on Saturday under an assumed name, somewhere in the southern United States, a senior American intelligence official said on Wednesday. He was 81.

In a statement with fittingly scant information, given Nosenko's earlier life as a KGB spy and his later life in the shadows, the senior official said he could provide no details about the cause of death or Nosenko's survivors, if any. The official himself would speak only on the condition of anonymity.

Only last month, several senior officials of the Central Intelligence Agency visited Nosenko to present him with an American flag and a letter from Michael V. Hayden, the director of central intelligence, thanking him for his service and, by implication, offering a final apology for the way he was treated after he defected to the United States in the winter of 1964.

Nosenko's defection seemed to have been motivated in part by his fondness for Western culture. He also said he needed money to repay some KGB money he had lost in Geneva after a night with a prostitute and a bottle of vodka in 1962. So he began spying in Moscow for the CIA, and eventually decided that his future lay in the United States.

He gave his American handlers vital information about Soviet agents who had penetrated American and European embassies, and about microphones that Russians had planted in the American Embassy in Moscow.

Most important, he said he had gone over the Soviet file on Lee Harvey Oswald, who lived in the Soviet Union in the late 1950s and early 1960s before traveling to Mexico City and then to Dallas, where President Kennedy was assassinated on Nov. 22, 1963.

At the time of Nosenko's defection, the Warren Commission was trying to determine whether Oswald, the presumed assassin, had acted on his own. Nosenko assured his American questioners that Oswald had never been an agent of the KGB, which had considered him unstable and unfit for espionage work. (Nosenko was not mentioned in the commission report.)

But instead of being relieved to hear that the Soviets had not been involved in the assassination, James Jesus Angleton, the CIA's legendarily suspicious counterintelligence chief, and others in the spy trade thought Nosenko's apparent defection was a trick. After all, the agency had suffered a series of setbacks, including the unmasking and execution of two Russian intelligence officials who had been spying for the CIA inside the Soviet Union.

"In the spring of 1964, after years of crushing failures, Angleton sought redemption," Tim Weiner, a reporter for The New York Times, recounted in his 2007 book, "Legacy of Ashes: The History of the CIA" (Anchor Books). Angleton "believed that if the CIA could break Nosenko, the master plot might be revealed — and the Kennedy assassination solved."

In fact, Nosenko said and did things to arouse suspicions about himself. Had he really been a lieutenant colonel in the KGB, as he initially claimed, or just a captain, as a senior intelligence official said on Wednesday? Had he really been so unfeeling as to be able to leave behind a wife and two young daughters, assuming the family really existed?

And had he really been disillusioned with the Soviet system, which had treated him quite well? His father, Ivan, was a naval engineer who became minister for shipbuilding under Nikita Khrushchev, and the young Yuri Ivanovich Nosenko had private tutors before graduating from the State Institute of International Relations in Moscow and moving on to the KGB.

In 1964, the CIA put Nosenko in solitary confinement at Camp Peary, its training site near Williamsburg, Va., where he got "the treatment his fellow Russians received in the gulag," as Weiner wrote.

"There were scanty meals of weak tea and gruel, a single bare light burning twenty-four hours a day, no human companionship," he wrote.

In a statement declassified in 2001, according to Weiner, Nosenko said: "I had no contact with anyone to talk. I could not read. I could not smoke. I even could not have fresh air."

After numerous lie-detector tests and many interrogation sessions, the CIA determined that Nosenko was telling the truth. He was released in 1967, given $80,000 and a new name and sent to spend the rest of his life somewhere in the South, with occasional trips to Langley, Va., to lecture American intelligence professionals at CIA headquarters.

Oleg D. Kalugin, a former senior officer in the KGB who is now an American citizen and a counterintelligence and security consultant, said on Wednesday that the suspicions of Angleton and others about Nosenko's sincerity had shown a "complete ignorance" about the KGB and Soviet values.

"It would have been unthinkable" for Moscow's leaders to have a KGB officer become a "mole" in the West, Kalugin said, because the apparent treason would have been so disillusioning to the Russian people that any intelligence he gleaned would not have been worth it.

But all these years later, there are doubters. Prominent among them is Tennent H. Bagley, who was one of Nosenko's main handlers as chief of counterintelligence for the CIA's Soviet division. "This KGB provocateur and deceiver," Bagley called Nosenko in his book "Spy Wars," published in 2007 by Yale University Press.

Claire George, a former CIA deputy director of operations, told The Washington Post, which first reported Nosenko's death on Wednesday, that Nosenko's treatment "was a terrible mistake." But, he added, "you can't be in the spy business without making mistakes."