Russian spy Sergei Skripal may have been poisoned with military grade nerve agent while driving his car in Salisbury, it is thought.
The former double agent and his daughter are both believed to have been in the red BMW at around 1.45pm on Sunday, March 4 - two hours before they collapsed in a park.
After the car was recovered and a supermarket car park was closed off, it appears the police investigation into nerve agent attack is now focusing on the vehicle.
The UK's head of counter-terrorism policing Neil Basu announced today that 38 people had come forward with illnesses related to the attack. As well as Mr Skripal, his daughter Yulia and DS Nick Bailey, who are all in hospital, another person is being monitored at home.
Speaking at Scotland Yard, he explained that the investigators' 'prime focus' is how the poison was administered, but warned the investigation will take 'many weeks'.
The Prime Minister told MPs yesterday that the highly dangerous substance used in the attack was a military-grade Novichok nerve agent produced by Russia.
Mrs May said the attempted murder of Sergei Skripal and his daughter, which also left policeman Detective Sergeant Nick Bailey seriously ill, was an 'indiscriminate and reckless act against the United Kingdom'.
Chemical warfare experts locked down a Wiltshire village yesterday amid fears that Sergei Skripal's car could contain traces of the nerve agent that's left him fighting for his life.
Army and police vehicles and ambulances rolled into West Winterslow just after lunchtime before investigators in hazardous material suits and gas masks loaded a white transit van onto a flatbed truck. The van's driver was also asked to had over his clothes, it was claimed.
Police also closed off a Sainsbury's car park where it is thought Mr Skripal's car was parked. A parking ticket machine was closed off and a police vehicle was removed from the scene by the Army.
Locations around the town, including a restaurant, a pub Mr Skripal's home, a car recovery yard and the park where the Skripals were found.
Theresa May has set a deadline of midnight tonight for Moscow to explain whether it was behind the attack or had lost control of its stockpile of the poison.
Failure to provide a 'credible' response would lead her to view the incident as 'an unlawful use of force by the Russian State against the United Kingdom', sparking unspecified measures in reprisal.
Mrs May said the government would not accept such an attempt to 'murder innocent civilians on our soil'.
She will decide on a range of sanctions over the next 24 hours after urgent talks with Nato, the United Nations, EU and US.
The Prime Minister will also draw up a secret package of measures against Russia which will never be revealed.
This could include targeting the Kremlin's propaganda machine. Whitehall sources said yesterday they were accelerating their offensive cyber programme and could hit select targets for a specific effect.
Home Secretary Amber Rudd, speaking after a meeting of the Government's emergency Cobra committee, today said: 'The investigation is going well. It is detailed and painstaking work with over 250 police officers in the area conducting this work.
'My priority is this incident, the investigation itself, so we get the information as quickly as possible, and also the safety and security of the people in the community, which is why we have such close co-ordination with the Department of Health and Public Health England.
'The Russians have started responding. The Prime Minister has been very clear that they have until midnight tonight to satisfy her requests. Until then we will wait and see what they have put forward.
'I know that international allies have begun to rally their support and make comments publicly but at the moment what we are doing is awaiting the Russian response before stepping up and responding as the Prime Minister has said we will.'
Vil Mirzayanov said the effects of the poison could amount to 'torture' - and claimed that the use of the lethal toxin on former spy Sergei Skripal and his daughter Yulia was a 'brazen attack'.
The chemist hit the headlines in 1992 after alleging in a newspaper article that Russia had been developing a particularly lethal new nerve gas.
Mr Mirzayanov, who was later arrested but now lives in the United States, had worked in a secret laboratory which was developing the Novichok nerve agents.
He said that the toxins were 'for paralysing people', adding: 'It causes you convulsions and you can't breathe and after that you die. If you get enough of a dose of it.
'It's real torture, it's impossible to imagine. Even in low doses the pain can go on for weeks. You cannot imagine the horror, it's so bad.'
News agency Tass quoted Russian foreign ministry spokeswoman Maria Zakharova as saying: 'It is a circus show in the British Parliament.
'The conclusion is obvious, it's another political information campaign, based on a provocation.'
Mr Putin dismissed questions about the Skripals when he was confronted during an election campaign visit, telling the BBC: 'Get to the bottom of things there, then we'll discuss this.'