© Joel Goodman / Global Look Press
Two people have been treated for "suspected exposure to an unknown substance" in the city of Salisbury, England, according to police. One of them is reportedly a former Russian double agent.

Sources close to the investigation told BBC and Reuters that one of those affected was Sergey Skripal, part of a "spy swap" between the US and Russia in 2010. The Russian worked as a double agent for the UK intelligence agency MI6 and was jailed in Russia in 2006 for spying for Britain, having passed on the names of undercover Russian intelligence agents. Russia released four prisoners in exchange for 10 US spies.

Police declared a "major incident" after a man and woman were reported to be in distress at a shopping center in Salisbury, in Wiltshire in southern England on Sunday. They "were taken to Salisbury District Hospital and are being treated for suspected exposure to an unknown substance. They are currently in a critical condition," Wiltshire Police said in a statement on Monday.

"At this stage it is not yet clear if a crime has been committed and a multi-agency response has been co-ordinated. Police are carrying out a full investigation and working with partner agencies, to clarify the exact circumstances," police said, adding that at this stage they don't believe there is any risk to the wider public.

"SalisburyNHS is currently dealing with a major incident involving a small number of casualties, with a multi-agency response," Salisbury Hospital said. "We are not asking additional staff to come to site unless contacted directly."

A hospital spokesman told the Sun that the incident involved a small number of casualties. "It involves under ten people, but I cannot say any more," he said.

A Public Health England (PHE) spokesman said those exposed to the substance have been "decontaminated." It added that, "scientists from PHE's Centre for Radiation, Chemical and Environmental Hazards, will continue to assist the response and review information as it becomes available."

"This has not been declared as a counter-terrorism incident and we would urge people not to speculate," Wiltshire Police's Temporary Assistant Chief Constable Craig Holden told reporters. "However, I must emphasize that we retain an open mind, and that we continue to review this position."

The Salisbury Journal reported that emergency services suspected fentanyl, a synthetic drug similar to heroin, only far stronger, may have been involved. It isn't clear whether this indicates the pair had taken the drug or merely been exposed to it; because of fentanyl's potency, it can even endanger those who come into contact with it, as it can be absorbed through the skin.

An eyewitness told the BBC she saw an older woman and a man sitting on a bench and that they "looked like they'd been taking something quite strong." She said the woman appeared to be collapsed against the man, who was making strange hand gestures and looking up at the sky. "They looked so out of it that I thought even if I did step in, I wasn't sure how I could help."

The British media rushed to compare this case with an incident involving former Russian security officer Alexander Litvinenko, who died from radioactive poisoning in 2006. His death sparked a major crisis in British-Russian relations, as many public figures in the West accused the Russian government of being involved. This time, however, no specific information has been released by police so far.