A Russian national living in the U.K. after his 2010 release by Moscow on charges of spying for Britain, is critically ill after being exposed to an "unknown substance" over the weekend, the BBC reports.
Sergei Skripal, 66, fell ill in a Salisbury, Wiltshire, shopping mall on Sunday.
In 2006, Skirpal, a retired military intelligence colonel, was jailed for 13 years after he was convicted of handing over to MI6 the identities of Russian spies working undercover in Europe during the 1990s. Russian prosecutors charged that he had taken tens of thousands of pounds from the British spy agency.
He was one of a handful of prisoners released by Russia as part of a swap in 2010.
A woman accompanying Skripal on Sunday, who is unidentified but thought to be in her 30s, also fell ill at the same time and is being treated at a local hospital.
"The pair, who we believe are known to each other, did not have any visible injuries and were taken to Salisbury District Hospital," Wiltshire Police temporary Assistant Chief Constable Craig Holden said. "They are currently being treated for suspected exposure to an unknown substance. Both are currently in a critical condition in intensive care."
The Telegraph reports: "It is thought the pair may have been exposed to the powerful synthetic drug, Fentanyl, which is up to 10,000 times more powerful than heroin and has been linked to scores of deaths in the UK."
According to the BBC:
"Police are investigating whether a crime has been committed, following the incident which began at 16:15 GMT on Sunday at the Maltings shopping centre in central Salisbury.
An area around the Maltings shopping centre and several other parts of the city were cordoned off as teams in full protective gear used hoses to decontaminate the street."
However, Holden said it was "still at the very early stages of the investigation [and] we are unable to ascertain whether or not a crime has taken place."