Ten people pleaded guilty Thursday to working in the U.S. as Russian secret agents, setting up what is expected to be the largest Russia-U.S. spy swap since the Cold War.
A U.S. official says the four had to sign oath admitting guilt, before being released, but Russian news reports say the Russian president has pardoned the spies involved with the swap.
According to the letter, the four are imprisoned in Russia for alleged contact with Western intelligence agencies.
The 10 defendants were expected to be deported to Russia within hours. One of the Russian prisoners expected to be released, a Russian arms control analyst, was reportedly plucked from a Moscow prison and flown to Vienna earlier Thursday.
A swap would have significant consequences for efforts between Washington and Moscow to repair ties chilled by a deepening atmosphere of suspicion.
The U.S. defendants each pleaded guilty to conspiracy to act as an unregistered agent of a foreign country. An 11th defendant remains a fugitive after he fled authorities in Cyprus following his release on bail.
The arrests occurred more than a week ago, capping a decade-plus investigation of people who seemed to have embedded themselves in the fabric of American life. Authorities said they were reporting what they learned in the U.S. to Russian officials.
One person familiar with the plea negotiations told The Associated Press that most of the defendants expected to be going home to Russia later Thursday. The person was not authorized to publicly discuss the matter in advance of the plea and spoke on condition of anonymity.
Igor Sutyagin, the Russian arms control analyst, was serving a 14-year sentence for spying for the United States. He had told relatives he would be freed in exchange for the 11 people charged in the United States with being Russian agents.
In Moscow, his lawyer, Anna Stavitskaya, said a journalist called Sutyagin's family to inform them that Sutyagin was seen walking off a plane in Vienna on Thursday. However, she told the AP she couldn't get confirmation of that claim from Russian authorities.
Russian and U.S. officials have refused to comment on any possible swap.
U.S. Attorney Preet Bharara would say Thursday only that prosecutors strive in all cases "to make sure that justice is served if consistent with the needs of national security, and the way we deal with national security is to make sure that is done in a way that is consistent with justice.
"Whatever the disposition is in this case, I think people should be confident it was done in the interest of national security and justice," Bharara said in White Plains, N.Y.
Candidates that Russia was reported to be offering in a spy swap were Igor Sutyagin, a military analyst with the U.S.A. and Canada Institute; Sergei Skripal, a former colonel in the Russian military intelligence; Alexander Sypachev, a colonel in the Russian intelligence service, and Alexander Zaporozhsky, a former colonel in the Russian Foreign Intelligence Service.
Sutyagin was sentenced to 15 years in prison in 2004 on charges of passing information on nuclear submarines and other weapons to a British company that Russia claimed was a CIA cover. Skripal was found guilty of passing state secrets to Britain and sentenced to 13 years in 2006. Sypachev was sentenced in 2002 to eight years in prison on charges of passing secrets to the CIA. Zaporozhsky was sentenced in 2003 to 18 years in prison for espionage on behalf of the United States.
Fox News' Jim Angle and the Associated Press contributed to this report.