Sergei Skripal, a former Russian spy who is critically ill in the UK after exposure to an unknown substance, was one of four prisoners who were pardoned and released from custody in 2010 as part of a spy swap.
The exchange followed the exposure of a ring of Russian sleeper agents in the US.
A look at who the four former prisoners were:
Skripal, a former Russian military intelligence colonel, was found guilty of passing state secrets to Britain and sentenced to 13 years in prison in 2006. He was accused of revealing the names of several dozen Russian agents working in Europe.
He served with Russian military intelligence (GRU) and retired in 1999 with the rank of colonel. He then worked at the Foreign Ministry until 2003 and later became involved in business.
Following his arrest in Moscow in December 2004, he confessed that he was recruited by British intelligence in 1995 and was feeding them information about GRU agents in Europe, receiving more than 100,000 dollars for his services.
At the time of his trial, the FSB domestic security agency was quoted as saying that his activities were as damaging to Russia as the work of Oleg Penkovsky, a GRU colonel who spied for the US and Britain and was executed in 1963.
Prosecutors asked for a 15-year prison sentence for Skripal, but the court took into account his co-operation with investigators and handed him a 13-year sentence.
He moved to Britain after his 2010 release and kept a low profile.
Sutyagin, a former military analyst with the USA and Canada Institute, a respected Moscow-based think tank, was sentenced to 15 years in 2004 on charges of passing information on nuclear submarines and other weapons to a British company that Russia claimed was a CIA cover.
During a highly publicised trial he insisted on his innocence, saying the information he provided was available from open sources. Amnesty International named him a prisoner of conscience.
Like Skripal, he moved to Britain after his release. He was hired by the Royal United Services Institute think tank in London where he works as a senior research fellow, focusing on nuclear and conventional arms control and the Russian arms programme.
He said in an interview with Russian television in 2011 that his family had stayed behind and he was reluctant to leave Russia but had no choice. He said he signed a confession and agreed to be part of the swap out of concern he would otherwise ruin everyone else's chances - and for fear of abuse and misery in the three years remaining in his prison term.
A former colonel in the Russian Foreign Intelligence Service, he was sentenced in 2003 to 18 years in prison for espionage on behalf of the US.
Zaporozhsky quit the service in 1997 and moved to suburban Baltimore in 2001. He was arrested after he returned to Moscow for what he thought was a reunion with KGB colleagues.
Russian media reported that he may have provided information leading to the capture of Robert Hanssen and Aldrich Ames, two of the most damaging spies caught in the US.
The former KGB officer employed as a security officer by Russia's NTV television was arrested in 2005. In 2006, he was sentenced to three years on murky charges of illegal weapons possession and resistance to authorities.
Russian media reported that he served in a counter-intelligence unit and was posted to the US. He was arrested on an assignment in Havana in 1988 and taken to Moscow, but was released six months later for lack of evidence. Suspicions lingered and he was arrested again in 2006 in Moscow.
Vasilenko has a home in Virginia.