CNN  — 

A woman found unconscious on a shopping center bench in southern England next to a Russian former double agent is his daughter, a source told CNN on Tuesday.

Sergei Skripal, 66, a former Russian military official convicted of spying for the UK, and his daughter, Yulia, 33, who was visiting from Russia, are critically ill in a UK hospital after “suspected exposure to an unknown substance.”

British counterterrorism police are investigating how the pair ended up slumped on a bench Sunday in Salisbury, England.

Sergei Skripal, here in a Moscow courtroom in 2006, was convicted in Russia of spying for Britain.

The incident has not been declared terrorism. But due to the unusual circumstances, the specialized counterterrorism unit will lead the investigation, London’s Metropolitan Police said Tuesday.

Skripal was convicted in Russia of spying for Britain before being granted refuge in the UK after a high-profile spy swap between the United States and Russia in 2010.

A forensic tent stands over a bench were Skripal and his daughter were found in Salisbury, England.

Salisbury, perhaps best known as the stopping-off point for tourists visiting nearby Stonehenge, has become the unlikely center of an extensive police probe into the mysterious circumstances around how Skripal and his daughter fell ill.

The pair were found on a bench in an unremarkable outdoor shopping complex, surrounded by a handful of chain eateries, including the Italian restaurant Zizzi, which was closed as part of the inquiry.

They did not have any visible injuries, according to police.

Yulia Skripal  is thought to be one of the few members of the former spy's immediate family still alive.

Images taken outside the restaurant Monday night showed investigators in anti-contamination suits searching the premises. Authorities have declined to name the substance to which the pair were exposed.

A small number of emergency services personnel were treated immediately afterward, local police said Tuesday. One of them remains in the hospital.

Investigators leave the park bench in Salibury covered with a forensic tent Tuesday.

Salisbury resident Freya Church described seeing the pair on the bench appearing “out of it.”

“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 told Reuters on Monday.

“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.”