A man and woman found unconscious in Wiltshire were poisoned by Novichok, the same nerve agent as 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 on Saturday.
Police say no one else has presented with the same symptoms.
There was "nothing in their background" to suggest the pair were targeted, the Met Police said.
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.
However he said the possibility the two investigations were linked 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 their movements" in order to determine where the pair had been exposed to the substance.
"We [the public] shouldn't be picking up anything that we have no idea what it is because we have no idea what may have contained the nerve agent at this time," he added.
The Counter Terrorism Policing Network is now leading the investigation, working with Wiltshire Police.
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.
"You do not need to seek advice from a health professional unless you are experiencing symptoms, as any individual who had been significantly exposed at the same time would by now have symptoms."