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.”
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.