Police are trying to identify a substance which caused a former Russian agent convicted of spying for Britain to collapse in Salisbury, Wiltshire.
Sergei Skripal, 66 – who was granted refuge in the UK in 2010 as part of a “spy swap” – and a 33-year-old woman are critically ill in hospital.
Meanwhile police have closed the city’s Zizzi restaurant “as a precaution”.
The pair were found slumped unconscious on a bench at the Maltings shopping centre on Sunday.
Wiltshire Police said the pair had no visible injuries and officers were investigating whether a crime had been committed.
Temporary Assistant Chief Constable Craig Holden said: “They are currently being treated for suspected exposure to an unknown substance…
“The focus is trying to establish what has caused these people to become critically ill.
“We are working with partners to prioritise this diagnosis and ensure that they receive the most appropriate and timely treatment.”
He said the police’s “major incident” response was not a counter-terrorism investigation.
But he said multiple agencies were involved and police were keeping an “open mind”.
Col Skripal, who is a retired Russian military intelligence officer, was jailed for 13 years by Russia in 2006.
He was convicted of passing the identities of Russian intelligence agents working undercover in Europe to the UK’s Secret Intelligence Service, MI6.
He was one of four prisoners released by Moscow in exchange for 10 US spies as part of a swap and was later flown to the UK.
He and the woman, who police said were known to each other, are both in intensive care at Salisbury District Hospital.
A number of locations in the city centre were cordoned off and teams in full protective gear have used hoses to decontaminate the street.
Workers in respirators and hazardous material suits searched bins close to the scene where the two collapsed.
On the restaurant closure, police said Public Health England had reiterated there was no known risk to the wider public.
As a precaution, they advised that if people felt ill they should contact the NHS on 111, or ring 999 “if you feel your own or another’s health is significantly deteriorating”.
Neighbours at Col Skripal’s home in Salisbury said police arrived around 17:00 GMT on Sunday and had been there ever since.
They said he was friendly and in recent years had lost his wife.
An eyewitness to the scene where the pair were found, Freya Church, told the BBC she saw them sitting on the bench: “An older guy and a younger girl. She was sort of leant in on him, it looked like she had passed out maybe.
“He was doing some strange hand movements, looking up to the sky…
“They looked so out of it I thought even if I did step in I wasn’t sure how I could help.”
By BBC security correspondent Gordon Corera
The parallels are striking with the 2006 case of Alexander Litvinenko.
He, too, was a former Russian intelligence officer who had come to the UK and was taken ill for reasons that were initially unclear.
In that case, it took weeks to establish that the cause was deliberate poisoning, and it took close to a decade before a public inquiry pointed the finger of blame at the Russian state.
Officials are stressing that it is too early this time to speculate on what happened here or why.
The police are not even yet saying a crime has been committed, but if the similarities do firm up and Moscow is once again found to be in the frame there will be questions about what kind of response might be required – and whether enough was done in the past to deter such activity being repeated.
The possibility of an unexplained substance being involved has drawn comparisons with the 2006 poisoning of Alexander Litvinenko.
The Russian dissident and former intelligence officer died in London after drinking tea laced with a radioactive substance.
A public inquiry concluded that his killing had probably been carried out with the approval of the Russian President, Vladimir Putin.
A spokesman for the Russian Embassy in the UK, when asked for comment on the Salisbury incident, said: “Neither relatives nor legal representatives of the said person, nor the British authorities, have addressed the embassy in this regard.”
Mr Litvinenko’s widow, Marina Litvinenko, told BBC Radio 4’s The World Tonight the latest incident felt like “deja vu” – and called for those receiving political asylum to be “completely safe”.
She said: “It just shows how we need to take it seriously, all of these people asking for security and for safety in the UK.”
Former Foreign Secretary Sir Malcolm Rifkind told The World Tonight the police approach in this case suggested there could be a “very sinister background”.
He said: “It could indeed potentially have been the FSB [Russian intelligence services] or the Kremlin could have been behind it.
“It could have been some form of criminal response for other reasons, or it could be some form of personal grievance some individual had against these two people or either of them.
“We don’t know at this stage and it is not going to be useful to speculate beyond that,” he added.
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