Sergei Skripal is a retired Russian military intelligence colonel who was sentenced in 2006 to 13 years in prison, accused of spying for Britain.
He was convicted, and later pardoned, for passing the identities of Russian secret agents in Europe to the UK’s Secret Intelligence Service (MI6).
Russia claimed MI6 had paid him $100,000 for the information, which he had been supplying since the 1990s.
But he was one of four prisoners Moscow swapped for spies in the US in 2010.
Col Skripal, now believed to be 66, was later flown to the UK.
He was found on Sunday slumped on a shopping centre bench in Salisbury, alongside his unconscious 33-year-old daughter Yulia Skripal.
The pair are critically ill in intensive care, and local authorities are investigating their exposure to “an unknown substance”.
A police source told the BBC that two officers dealing with the suspected poisoning in Salisbury were admitted to hospital on Monday after suffering ‘minor symptoms’, including itchy eyes and wheezing.
They were later released but another emergency responder is still in hospital. Public Health England says there is no outstanding health risk.
What were the charges against him?
Col Skripal was well regarded during his career in Russia’s military intelligence (GRU).
A colleague who worked alongside Col Skripal called him “the life and soul of the party” and added: “All his colleagues respected him. So when he was arrested for spying, it was a real shock.”
He was arrested near his home in 2004 and convicted two years later of “high treason in the form of espionage” by Moscow’s military court. He was stripped of all his titles and awards.
Sergei Skripal, 66, had been living in Salisbury after being released by Russia in 2010
He was alleged by the Russian security service (FSB) to have been recruited in 1995 for the British secret services while serving in the army.
He exchanged information classified as Russian state secrets with MI6 for money to be transferred to a Spanish bank account, the FSB claimed.
The data on “several dozen” Russian undercover operatives, according to pro-Kremlin daily Izvestiya, was eventually used to put FSB agents under surveillance and then later expel them from multiple European countries.
Even after his retirement in 1999, the FSB claimed Col Skripal continued to pass on state secrets.
Col Skripal pleaded guilty at his trial and allegedly confessed while co-operating with investigators, reports said at the time.
Russian media – both government-controlled and not – gave a lot of air time to Col Skripal’s trial and sentencing. They noted that his punishment of 13 years was a light sentence considering the “substantial damage” he caused to Russian intelligence services.
The daily newspaper Komosolskaya Pravda opined that Col Skripal “got off lightly” and that in Soviet times he would have been shot.
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What about the prisoner swap?
In July 2010, Col Skripal was pardoned by Russia’s then-President Dmitry Medvedev.
He was later released together with three other individuals serving time in Russian prisons in exchange for 10 Russian spies arrested by the FBI.
Among those released by the US was Anna Chapman, who had previously lived in the UK and gained citizenship.
Anna Chapman went on to work on Russian TV
Dubbed by the media as “Russia’s most glamorous secret agent”, she went on to present a weekly show on Russian TV, worked as a model and was employed by the Russian government as head of a youth council.
Deaths in the family
After a Cold War-style spy swap at Austria’s Vienna airport, Col Skripal was given refuge in the UK, where he kept a low profile for eight years.
His wife Liudmila died of cancer in 2012, a year after they had bought a semi-detached home in Salisbury. The Skripals had chosen the Wiltshire city because they believed it to be a good area with a low crime rate, family friends told the BBC.
Two years ago, Col Skripal’s older brother died in Russia, and last year, his 43-year-old son Alexander died while on holiday with his girlfriend in the Russian city of St Petersburg. He had been rushed to hospital with liver failure.
Family members were suspicious at the time and still believe some of the deaths were under mysterious circumstances.
Both Mrs Skripal and her son are buried in Salisbury.
Col Skripal’s daughter Yulia, who was found unconscious next to her father, was visiting from Moscow, relatives told the BBC.
The 33-year-old lived with her parents and brother in Salisbury after 2010 for several years before returning to the Russian capital because she reportedly missed it.
She and her brother were able to travel freely between the UK and Russia, despite their exiled father.
Yulia increased her visits to her father after her brother’s death last July. His next birthday would have been last week.
Since settling in Salisbury after the spy swap in 2010, Col Skripal appeared to be leading a relaxed and quiet life although he reportedly went on frequent business trips, the BBC’s World Affairs Correspondent Richard Galpin said.
Col Skripal reportedly kept the company of British intelligence agents and was a member of Salisbury’s Railway Social Club.
Valery Morozov, another Russian exile living in the UK, said Col Skripal told him he had been doing business, “working mainly in cyber-security”.
Mr Morozov said: “I understand he was working for some Russian groups.”
Col Skripal last called his mother, who is very ill, two weeks ago. He reportedly sounded optimistic, though his family say after the 2010 spy swap, he was always very vigilant because he believed the Russian special services could come after him at any time.
Sergei Skripal’s family strongly deny that he was an agent of MI6, insisting he was a big patriot and the case against him was fabricated.