A former Russian military intelligence colonel convicted of spying for the UK appears to have been the victim of an assassination attempt in Salisbury, England on Sunday.

Sergei Skripal, 66, was convicted in Russian courts of passing the identities of Russian operatives in Europe and the UK to the secret intelligence agency MI6. He was sentenced to thirteen years in prison in 2006, but was eventually pardoned and released as a part of a prisoner exchange between the Kremlin and United States in 2010. Skripal was one of four operatives released by the Russian government in that exchange.

According to Wiltshire Police, they responded to a call on Sunday afternoon that involved a man and woman taking ill after being exposed to an unknown substance. Although the type of substance the two were exposed to has not yet been revealed, law enforcement immediately cordoned off the hospital department they were being treated in, as well as portions of the city center where the exposure may have taken place. Currently, it would appear that the initial exposure took place at The Maltings shopping center in central Salisbury.

Teams of people in HAZMAT gear have been seen using hoses to decontaminate the cordoned off areas in the city center that may have been exposed to whatever the substance was.

Neither police nor the hospital have offered details thus far, but a “major incident” has been declared at the Salisbury District Hospital as a result of the exposure. Hours after the event first hit the local news, it was revealed that the critically ill man was indeed Skripal, who had been granted refuge in the UK following the prisoner exchange in 2010.

The woman, who has yet to be identified, is reportedly in her mid-thirties and is also undergoing medical treatment in the newly secured portion of the hospital.

According to court documents from Skripal’s trial, the Kremlin accused him of accepting a total of $100,000 from MI6 over the span of years, beginning in the 1990s. He pled guilty and was convicted of “high treason in the form of espionage” by a military court and stripped of his rank and all awards, before being sent to a penal camp to fulfill his thirteen year sentence. Skripal served four years of that sentence before being released in the aforementioned prisoner exchange that led him to resettling in the UK.

He has been said to maintain a low profile in the years since his release. Neighbors reported that police arrived at his home in Salisbury at around 5 p.m. local time, less than an hour after the initial incident occurred, and that there has been a law enforcement presence there ever since.