British media is reporting that a former Russian spy is among two people in a critical condition after suspected exposure to an 'unknown substance' in England.
A man in his 60s and a woman in her 30s were found unconscious on a bench at a shopping centre in Salisbury on Sunday afternoon.
The two people, who are believed to be known to each other, did not have any visible injuries.
Police say the pair have been taken to a local hospital, and remain in intensive care being treated for suspected exposure to an unknown substance.
Officers say they have yet to discover "whether or not a crime has taken place".
Temporary Assistant Chief Constable Craig Holden of Wiltshire Police explained: "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.
"This has not been declared as a counter terrorism incident and we would urge people not to speculate. However, I must emphasise that we retain an open mind and we will continue to review this position.
He added: "We have access to a wide range of specialist resources and services that are helping us to understand what we are or aren't dealing with at this time."
Multiple British media outlets have named 66-year-old Sergei Skripal as the man being treated in hospital.
He was convicted in Russia in 2006 of passing state secrets to Britain, before later being given refuge in the UK as part of a spy swap.