A former Russian spy is in critical condition after what police in southwest England described as "suspected exposure to an unknown substance," a source familiar with the investigation told CNN.

A man in his 60s and a woman in her 30s were found unconscious on a shopping center bench in Salisbury, Wiltshire police said Monday night. The pair knew each other but their identities have not been confirmed by the UK Home Office, Wiltshire Police or Salisbury Hospital.

A source familiar with the investigation identified one of the two people as Sergei Skripal, who was granted refuge in the UK following a "spy swap" between the US and Russia in 2010.

The two are being being treated at Salisbury District Hospital's intensive care unit, where their condition remains critical.

Temporary Assistant Chief Constable Craig Holden said the incident is not being treated as a terrorism incident and urged people not to speculate.

Police are keeping an open mind regarding the cause of injuries, he said. They do not believe that there is any risk to the wider public, he added. Police are working with partners to prioritize the couple's diagnosis.

CNN's Max Foster contributed to this report.