Former US intelligence contractor Edward Snowden's stop in Moscow is an unexpected windfall for the Russian secret services even if it risks worsening the already strained relations between the Kremlin and Washington.
Snowden's sensational revelations on the electronic surveillance carried out around the world by the United States present great interest for Russian secret services, who undoubtedly have questioned him in Moscow, experts said Monday in Moscow.
The location of the fugitive is now a mystery. He was not present on Monday's flight to Cuba on which he was booked and his current whereabouts have not been confirmed amid reports he has already left Russia.
"Russian intelligence and counter-intelligence will have a lot to ask such a well-informed person. I have no doubt that this will be done," a Russian special services veteran told the Interfax news agency on Sunday on condition of anonymity.
"I am sure that Snowden will have had a busy evening and a sleepless night," the source added of the American's reported stayover Monday night in the transit zone of Moscow's Sheremetyevo airport.
"Snowden presents a lot of interest for the FSB (the Russian security service). He can give information on technical aspects of intercepting data," said Russian security expert and commentator for the Novaya Gazeta newspaper Pavel Felgenhauer.
"A debriefing in the presence of technical specialists takes a lot of time," Felgenhauer told AFP, suggesting that interviews with Russian secret services could take place in a third country.
Since June 5, Snowden has exposed information on collecting data on phone calls and Internet communications by American intelligence agencies and there is little question that his knowledge could be valuable to the Russian secret services.
But whatever Russia gains from speaking to the man, on a diplomatic level the Snowden case will cause a deep-freeze in the already tense relations between Moscow and Washington.
While Snowden on Sunday was seemingly in the transit zone in Moscow, Washington asked Moscow to expel him and send him to the United States.
"We expect the Russian government to look at all options available to expel Mr. Snowden back to the US to face justice for the crimes with which he is charged," said National Security Council spokesperson Caitlin Hayden.
Moscow did not respond to this demand and apparently authorised Snowden to continue his route as he had checked in for an Aeroflot flight to Cuba.
"Relations between Russia and the United States are so bad that I don't know what you could do to make them worse," Viktor Kremenyuk, deputy director of the USA and Canada Institute, told AFP.
The United States had unsuccessfully made the same extradition demand to Hong Kong, from where the American apparently flew out to Moscow on Sunday.
The announcement of Snowden's departure from Hong Kong to Moscow by whistleblowers WikiLeaks, which was confirmed by Ecuador, caused a sensation in Russia, whose relations with the United States recently have recalled the Cold War years.
Russia had said several days before that it would consider it if Snowden made a request for asylum.
The Snowden case is yet another addition to a list of issues where Moscow and Washington take opposing positions, including the conflict in Syria and the deterioration of respect for human rights in Russia.
New York Senator Chuck Schumer told CNN on Sunday that "Putin always seems almost eager to put a finger in the eye of the United States," adding that Moscow's allowing Snowden to travel onward would have "serious consequences" for bilateral relations.