A former Russian spy was reportedly found in critical condition in the U.K. on Sunday amid suspicions he might have been exposed to an unknown chemical substance.

Ex-spy Sergei Skripal and an unidentified woman were found on a park bench in Salisbury, according to media reports.

A witness told the BBC that Skripal was looking up at the sky making strange hand movements, while the woman appeared to be unconscious and leaning on him.

The Guardian reported that fentanyl may have been used in the incident.

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Skripal was one of four Russians who were exchanged for 10 “sleeper” agents placed by the Kremlin in the U.S. in 2010.

Skripal was sentenced to 13 years in jail in Russia in 2006 after he gave the identities of Russian agents working in Europe to Britain's secret intelligence service, MI-6.

It was believed that Skripal received a new identity and home in the U.K.

Local authorities said the incident has not yet been labeled as a terror incident.

“This has not been declared as a counter-terrorism incident and we would urge people not to speculate," Wiltshire police temporary assistant Chief Constable Craig Holden said, according to The Guardian. “However, I must emphasize that we retain an open mind and we will continue to review this position.”

The incident has parallels to the poisoning of former Russian spy Alexander Litvinenko in London in 2006.

The poisoning put a major rift in relations between Moscow and London, despite denials from Russia that it was involved.