A retired Russian double agent was leading a seemingly peaceful life in rural Britain when he and his visiting daughter were targeted in a nerve agent attack.
Here is what we know about the Sergei Skripal affair.
Who is Sergei Skripal?
A former colonel in the Russian military intelligence service, Sergei Skripal was accused of "high treason" in 2006 for selling information to Britain and sentenced to 13 years in prison.
In 2010, he benefited from a spy swap between Moscow on the one side and London and Washington on the other.
In a highly unusual move, Russia agreed to release a double agent arrested in its territory, but received 10 Russian spies released by Washington in exchange.
His new life
The former agent settled in the small city of Salisbury, south-west of London, best known for its medieval cathedral.
Here he lived an apparently unremarkable existence, setting up home in a modest red-brick suburban housing estate.
A surveillance video of Skripal captured days before the attack showed him buying milk and bacon at his local shop, where he would come "a couple of times a week" to buy scratchcards, shop owner Ebru Ozturk told the Mirror.
But another Russian exile, Valery Morozov, told British media that he had not stopped his espionage activities and was working in cyber-security.
What happened on March 4?
Skripal and his daughter Yulia, who was visiting from Russia, had lunch at Italian restaurant Zizzi before heading to The Mill pub for a drink.
A few hours later, several witnesses spotted them on a bench near the Maltings shopping mall, visibly ill.
One of them had vomited, while Sergei Skripal had glazed eyes and was shaking his hands towards the sky.
After a witness called the police, the pair were immediately taken to hospital, where they remain in a critical condition.
The first police officer to come into contact with the pair was also hospitalised, and remains in a serious condition.
What substance was used?
Police moved quickly to take samples after isolating the area and closing the restaurant and pub where the father and daughter visited.
Experts at Porton Down laboratories have identified the substance used as a military-grade nerve agent "of a type developed by Russia", and part of a group of such agents known as Novichok, according to British Prime Minister Theresa May.
On Monday she gave Moscow until the end of Tuesday to "provide full and complete disclosure of the Novichok programme" to the Organisation for the Prohibition of Chemical Weapons.
It is the implementing body of the Chemical Weapons Convention, which entered into force in 1997 and has 192 member states.
May did not detail how the dose of the nerve agent was administered, telling lawmakers it remained under police investigation.
British media, which reported a table in Zizzi was so contaminated it had to be destroyed, has been rife with speculation on whether the poison was sprayed on the couple, secreted in food or drink or brought unknowingly by Yulia in a package.
Who is responsible?
May told British members of parliament on Monday that it was "highly likely" that Moscow was behind the poisoning.
"The Government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal," May said.
She added Britain had reached the verdict based on its knowledge that Russia had previously produced the agent and could still do so, the country's "record of conducting state-sponsored assassinations" and its analysis that Moscow views some defectors as legitimate targets for killing.
The case of Alexander Litvinenko, a Russian dissident who died of Polonium poisoning in a 2006 attack attributed to Moscow, is still fresh in the memory for many in Britain.
Foreign minister Boris Johnson, Britain's top diplomat, said he saw "echoes" of Litvinenko in the Skripal case, provoking anger in Moscow, which has repeatedly maintained its innocence in all the accusations.