(CNN) One of two people critically ill in a UK hospital after a "suspected exposure to an unknown substance" is a former Russian military official convicted of being a British spy, a source familiar with the investigation told CNN.

Sergei Skripal, 66, who was granted refuge in the UK after a high-profile " spy swap " between the US and Russia in 2010, was found slumped on a shopping center bench in Salisbury, southern England, with an unidentified woman. Police said both were unconscious when they were discovered on Sunday afternoon.

Authorities declined to confirm their identities, nor the nature of the substance to which they were exposed. Police described them only as a man in his 60s and a woman in her 30s, and were known to each other.

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

Wiltshire police temporary assistant Chief Constable Craig Holden said police were keeping an open mind on the cause of injuries. He said the incident was not being treated as terrorism and cautioned against speculation.

A branch of the Zizzi restaurant chain, close to where the couple were found, was closed as part of the inquiry. Images taken outside the restaurant Monday night show investigators wearing anti-contamination suits searching the premises.

In a statement , police said although they did not believe there was a risk to public health, anyone who felt ill should contact emergency services.

Police inside a Zizzi restaurant in Castle Street, Salisbury.

A woman who saw Skripal and the unnamed woman earlier Sunday described the couple as appearing "out of it." Freya Church, a local resident, said she saw the pair on a bench. "She sort of leant in on him, it looked like she'd passed out maybe. He was doing some strange hand movements, looking up to the sky," Church said.

"I felt like I should step in but to be honest they looked so out of it that I thought that even if I did step in I wasn't sure how I would help. So yeah, I just left them, but it looked like they'd been taking something quite strong."

The case has drawn inevitable comparisons to the poisoning of former Russian spy Alexander Litvinenko, who died a slow death after meeting a contact at a London sushi restaurant in 2006.

detailed UK inquiry later concluded that President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko.

Former spy

Skripal arrived in the UK as part of an elaborately choreographed spy swap conducted by the United States and Russia in which the two countries exchanged agents on chartered planes on the runway at an airport in Vienna, Austria.

Among the ten so-called Russian "sleeper agents" deported by the United States as part of the deal was Manhattan socialite Anna Chapman , who had previously lived in London.

Skripal was one of four Russians who traveled in the opposite direction, after being pardoned by President Dmitry Medvedev.

According to previous reports by Russia's state news agency RIA Novosti , Skripal was convicted to 13 years imprisonment for spying for the UK in 2006.

It quoted Russia's Federal Security Service (FSB) as saying Skripal, a former Russian military intelligence colonel, was convicted for "treason in the form of espionage" and that he had been recruited by Britain's MI6 intelligence service, following his arrest in December 2004.

The report said he had spied for Britain while serving as an intelligence officer during the 1990s and that he had continued to communicate with MI6 after his retirement in 1999.

Russian court officials said Skripal had received at least $100,000 for his collaboration with MI6, RIA Novosti reported.

According to the FSB, Skripal's "actions caused serious damage to the national defense and security." The intelligence service added that MI6 paid Skripal for the information in foreign currency, which was transferred monthly to his account in a Spanish bank.

Russian newspaper reports at the time of his conviction said that Skripal had shared information about dozens of his former colleagues operating in Europe undercover, in particular, their secret meeting venues, addresses and passwords.

The FSB said that Skripal had admitted his guilt and gave truthful testimony about his activities, which the court had taken into consideration in sentencing him.

Skripal is believed to have lived in the UK since his release from Russian custody in 2010.

Correction: This story has been updated to clarify when the couple were found.