WASHINGTON – A former top Russian spy who defected to the United States after running espionage operations from the United Nations, Sergei Tretyakov, has died in Florida, his wife and a friend said Friday. He was 53.

News of his June 13 death came the same day the United States and Russia completed their largest spy swap since the Cold War.

Tretyakov’s defection in 2000 was one of the most prominent cases involving Russia’s intelligence agency in the past decade. Tretyakov later said his agents helped the Russian government steal nearly $500 million from the U.N.’s oil-for-food program in Iraq.

His widow, Helen Tretyakov, told WTOP Radio in Washington he died of natural causes. She said she announced his death Friday to prevent Russian intelligence from claiming responsibility or “flattering themselves that they punished Sergei.”

Helen Tretyakov said her husband warned U.S. authorities when he defected that Russia was expanding deep-cover operations.

“He was aware that the part of the SVR budget for supporting illegals increased dramatically in the 1990s,” she told WTOP. The SVR is the Russian intelligence agency that succeeded the KGB after the Soviet Union collapsed in 1991.

However, she said there was no direct link between his information and the 10 people arrested last month as Russian spies near Boston, New York and Washington. “It wasn’t him who disclosed the names of these people,” she said.

She asked friends not to make the death public until the cause was determined, according to author Pete Earley, who wrote a 2008 book about Tretyakov. Earley said Friday that Helen Tretyakov told him her husband died of cardiac arrest at home. The medical examiner’s office in Sarasota County, Fla., said the autopsy report was pending.

“Sergei was called ‘the most important spy for the U.S. since the collapse of the Soviet Union’ by an FBI official in my book,” Earley wrote on his blog. “Unfortunately, because much of what he said is still being used by U.S. counterintelligence officers, it will be years before the true extent of his contribution can be made public — if ever.”

A private funeral was held three days after Tretyakov’s death, in keeping with Russian Orthodox tradition, and more than 200 people attended a service in the days after, Earley said.

Tretyakov was born Oct. 5, 1956, in Moscow. He joined the KGB and rose quickly to become second-in-command of Russia’s U.N. mission in New York between 1995 and 2000.

Peter Earnest, director of the International Spy Museum in Washington, called Tretyakov’s 2000 defection significant.

Russia’s spies in the United States would have come under Tretyakov’s purview, said Earnest, who spent more than 30 years in the CIA.

For up to a decade after Tretyakov’s defection, the FBI kept watch over 10 Russian agents as they tried to blend into American suburbia. They were arrested last week and swapped Friday in Vienna for four people convicted in Russia of spying for the United States and Britain.

“That does bring into mind the question: Is that the sort of information he might have shared with the U.S. authorities?” Earnest said.

Earley interviewed Tretyakov at length for the book “Comrade J.: The Untold Secrets of Russia’s Master Spy in America after the End of the Cold War.”

“He had a fabulous memory that he had sharpened as a KGB/SVR officer, and he refused to speculate or exaggerate when he discussed KGB/SVR operations,” Earley wrote.

In a 2008 interview promoting Earley’s book, Tretyakov said his agents helped the Russian government skim hundreds of millions of dollars from the oil-for-food program before the fall of Saddam Hussein in 2003. He said he oversaw an operation that helped Hussein’s regime manipulate the price of oil sold under the program, and Russia skimmed profits.

Tretyakov called his defection “the major failure of Russian intelligence in the United States” and warned that Russia, despite the end of the Cold War, harbored bad intentions toward the United States.

Tretyakov said he found it immoral to continue helping the Russian government.

“I don’t see any light at the end of the tunnel. I’m not very emotional. I’m not a Boy Scout,” he said. “And finally in my life, when I defected, I did something good in my life. Because I want to help United States.”