A man and woman found unconscious in Britain's Wiltshire were poisoned by Novichok - the same nerve agent that poisoned ex-Russian spy Sergei Skripal, police say.
The couple, believed to be Charlie Rowley and Dawn Sturgess, are in a critical condition having been found unconscious at a house in Amesbury, Wiltshire, on Saturday.
Police say no one else has presented with the same symptoms, according to a BBC report Thursday.
There was "nothing in their background" to suggest the pair were targeted, the Met Police said.
Amesbury is about eight miles from the Salisbury Russian spy poisoning site.
Metropolitan Police Assistant Commissioner Neil Basu said it could not be confirmed whether the nerve agent came from the same batch that Mr Skripal, and his daughter Yulia, were exposed to.
But he said the possibility was "clearly a line of enquiry".
Mr Basu said no contaminated items had yet been found, but officers were putting together a "very detailed examination of [the couple's] movements" in order to determine where they were poisoned."
He added that members of the public should not pick anything up if they don't know what it is.
"We have no idea what may have contained the nerve agent at this time," he said.
The Counter Terrorism Policing Network is now leading the investigation, working with Wiltshire Police.
The BBC's security correspondent Gordon Corera said: "The most likely hypothesis is that this is leftover Novichok from the attack on the Skripals back in March."
England's chief medical officer, Sally Davies, said: "I want to reassure the public that the risk to the general public remains low."
The Skripal episode meant officials had a "well-established response" in place, she said.
"As before, my advice is to wash your clothes and wipe down any personal items, shoes and bags, with cleansing or baby wipes before disposing of them in the usual way."
On Saturday, paramedics were called twice to the property in Amesbury - in the morning, after Ms Sturgess had collapsed, then later the same day, after Mr Rowley had also fallen unwell.
"It was initially believed that the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of drugs," Wiltshire Police said.
The news that Novichok was to blame was announced following analysis at the defence research facility at Porton Down, Wiltshire.
As a precautionary measure, sites in Amesbury and Salisbury, believed to have been visited by the couple before they fell ill, have been cordoned off.
There is no evidence to suggest either visited the sites that were decontaminated following the poisoning of Sergei and Yulia Skripal.
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