A man identified by British media as a former Russian spy and his companion remained in intensive care early on March 6 while British authorities launched an investigation into why they fell critically ill.
"Alongside our partner agencies, we are conducting some extensive enquiries to determine exactly what led to these two people falling unconscious and clarify whether or not any criminal activity has happened," Craig Holden, temporary assistant chief constable of the Wiltshire police, said.
Media have identified the man as Sergei Skripal, a former colonel in Russia's GRU military intelligence agency who was convicted of passing state secrets to Britain in 2006 but who was released from prison in a spy swap in 2010.
But while Skripal appeared to be the latest of a string of Russians who suddenly and mysteriously took ill or died in Britain in recent years, Holden said media should not assume his illness is the result of foul play.
"This has not been declared as a counterterrorism incident and we would urge people not to speculate," Holden said. "However, I must emphasize that we retain an open mind and that we continue to review this position."
Police said the man, who is in his 60s, and a woman in her 30s were found unconscious on a bench at a shopping center in Salisbury in western England.
They were taken to a Salisbury hospital and are being treated for "suspected exposure to an unknown substance," police said, as crews sealed off and hosed down the area where they were found.
Parts of the city center remained sealed off early on March 6 as emergency responders in hazardous material suits continued to canvas there.
Police vehicles were also seen at what was listed as Skripal's home in Salisbury.
The National Health Service said it had only limited information about the patients, but there "doesn't appear to be any further immediate risk to public health."
The substance has not been identified. But local media reported that emergency services suspect the powerful synthetic opiate fentanyl may have been involved.
"They looked like they'd been taking something quite strong," the BBC quoted an eyewitness as saying.
"She was sort of leant in on him. It looked like she had passed out maybe," the eyewitness said. "He was doing some strange hand movements, looking up to the sky."
Skripal was arrested in Moscow in December 2004 and convicted by Moscow's military court in August 2006 of "high treason in the form of espionage."
He was found guilty of passing the identities of Russian intelligence agents working undercover in Europe to Britain’s Secret Intelligence Service, or MI6, in return for $100,000.
Russia’s Federal Security Service (FSB) alleged he had begun working for MI6 while serving in the army in the 1990s.
Skripal pleaded guilty, reports said at the time.
Skripal's illness from exposure to or ingestion of an unidentified substance brought to mind the sudden illness and death of former FSB officer Aleksandr Litvinenko in London in 2006.
British investigators have accused Andrei Lugovoi, a member of Russia's lower house of parliament, of carrying out the poisoning of Litvinenko at a hotel in the center of the British capital.
They concluded that Litvinenko ingested the highly radioactive isotope polonium 210 while drinking tea with Lugovoi and his alleged accomplice, Dmitry Kovtun.
Moscow has dismissed the inquiry as "opaque" and "politically motivated." Kovtun and Lugovoi have denied involvement despite traces of polonium that British investigators say the two left across London.
Several Russia experts in Britain said they feared Skripal is the latest apparent victim of an assassination attempt.
But Igor Sutyagin, who was part of the same spy swap as Skripal in July 2010 and is now is a senior research fellow at the Royal United Services Institute in London, told AP that there is not enough evidence yet to point fingers at Moscow.
"There are lots of former security officers that deserted to the West," he said, urging caution until more is known. "It is necessary to balance this information."
With reporting by the BBC, Reuters, AP, AFP, and Press Association