Here is what we know about the Sergei Skripal affair
Here is what we know about the Sergei Skripal affair:
A former colonel in the Russian military intelligence service, Skripal was accused of "high treason" in 2006 for selling information to Britain and sentenced to 13 years in prison.
In 2010, he was part of a Russia spy swap with Britain and the United States. Moscow received 10 Russian spies released by Washington in exchange for Skripal and three others.
Skripal lived in the quiet city of Salisbury, southwest England.
Skripal and his daughter Yulia -- visiting from Russia -- drove into Salisbury city centre and parked at The Maltings shopping centre.
They had a drink at The Bishop's Mill pub and then ate lunch at Italian restaurant Zizzi, leaving around 3:35pm.
Emergency services were called to a bench at about 4:15pm, finding them in what police said was an "extremely serious condition".
They were immediately taken to hospital.
Police officer Nick Bailey, who came to assist them, was also hospitalised.
More than 50 people reported to hospital.
Sergei Skripal is in a critical but stable condition in Salisbury District Hospital. He is under heavy sedation and unable to communicate.
Yulia Skripal is in a stable condition. The BBC said she was conscious and talking.
Bailey was discharged on March 22.
British police believe the Skripals first came into contact with the nerve agent from their front door, where the highest concentration has so far been found.
Counter-terror detectives are undertaking one of their biggest-ever investigations, with 250 officers on the case.
They are going through more than 5,000 hours of security camera footage, 1,350 exhibits and 500 identified witnesses.
Britain's Porton Down defence laboratory identified the substance used as military-grade Novichok. Only state actors were capable of the extremely sophisticated production method, its chief said.
But the laboratory said it could not verify the precise source of the nerve agent.
The Organisation for the Prohibition of Chemical Weapons (OPCW) global watchdog is now in Britain conducting its own tests, including on the Skripals' blood.
Novichok -- or "newcomer" in English -- refers to a group of powerful and deadly chemical compounds reportedly developed by the Soviet government in the 1970s and 1980s, some of which were adopted by the Soviet army in 1990.
Almost everything in the public domain comes from Russian defectors who revealed the existence of the secret weapons programme in the 1990s.
Britain expelled 23 Russian diplomats it claimed were undeclared intelligence officers. The United States threw out 60, while 24 other European nations followed suit, plus Canada and Australia.
Russia has retaliated in kind, expelling diplomats and closing the US and British consulates in St. Petersburg.
The United States, France and Germany joined Britain in saying there was "no plausible alternative explanation" than Russian state responsibility.
The quartet say March 4 was the first offensive use of a nerve agent in Europe since World War II.
Britain gave Russia until March 14 to explain whether it was behind the attack or had lost control of its chemical weapons stockpile. Moscow did not agree to the request.
Britain now says there is "no alternative conclusion" than Russian state culpability.
It said the conclusion was based on the identification of Novichok, knowledge that Russia had produced Novichok, its record of "state-sponsored assassinations" and that it "views defectors as suitable targets for assassination".
Britain's official at the OPCW told the watchdog Wednesday that instead of answering London's question, Russia had provided "more than 24 contradictory and changing counter narratives".
Russia denies all suggestions that the attack could have had anything to do with the Kremlin. Moscow claims it is the victim of Russophobia. Russian officials have offered a variety of possible theories.
Russia's foreign ministry has suggested that Britain, Sweden, the Czech Republic and Slovakia were "the most likely source of this chemical attack".
On March 22, Russia's ambassador to Britain questioned whether it was a "coincidence" that the attack happened eight miles (13 kilometres) from Porton Down.
On Wednesday, the head of Russia's SVR foreign intelligence agency said the poisoning was a "grotesque provocation... crudely concocted by the British and American security services".