A newspaper in Moscow has named the Russian intelligence agent it claims helped America break up a Russian spy ring last summer.
The paper, Kommersant, says a Col Shcherbakov of the Russian foreign intelligence agency had been working for the Americans.
Ten Russian sleeper agents were arrested and sent back home, in the biggest spy swap since the Cold War.
It was the biggest US-Russian spy scandal since the end of the Cold War.
Col Shcherbakov had a senior role in Russia's foreign intelligence agency, Kommersant says; his job: to plant moles in the United States, secret agents deep under cover.
But at some point the colonel changed sides.
Quoting intelligence sources, the newspaper says Col Shcherbakov fled to America in June, just three days before President Dmitry Medvedev's official visit to the US.
A few days after that, once the Russian president was back in the Kremlin, the Russian agents were seized.
Kommersant cited sources who claimed Col Shcherbakov visited one of the arrested spies, Mikhail Vasenkov, in prison to try to convince him to confess.
But the spy, who went by the alias Juan Lazaro, insisted he was not Russian.
Col Shcherbakov then presented Mr Vasenkov's dossier to US officials, according to the newspaper.
A year before the spy scandal, Col Shcherbakov had reportedly been offered a promotion at work, but had turned it down, to avoid having to take a lie detector test.
The newspaper claims Col Shcherbakov's bosses had overlooked the fact his daughter had been living in America for years.
Kommersant quotes an unidentified Kremlin official as suggesting that a Russian hit squad was already planning to kill the colonel.
The Kremlin source predicted Col Shcherbakov would spend the rest of his life fearing retribution.
A spokesman for Russia's foreign intelligence agency refused to comment on the newspaper report.
But Gennady Gudkov, deputy chairman of the Russian parliament's security committee, confirmed that Kommersant's report was correct.
"Shcherbakov turned over our agents in the USA - I knew of this long before the publication today in Kommersant," Mr Gudkov said.
"It is a major blow to the image of the Russian intelligence services," he told Reuters news agency.