SALISBURY, England -- British authorities are trying to figure out what sickened a former Russian spy who's now fighting for his life, and counter terrorism police have taken over the investigation. Sergei Skripal is a former Russian army colonel and spy who moved to the U.K. in 2010. On Tuesday, he lay in a Salisbury hospital, critically ill after exposure to what police say is a so far unknown substance, reports CBS News correspondent Elizabeth Palmer.

A couple seen walking on security camera video are believed to be Skripal and a woman identified as his daughter Yulia, 33. They are pictured walking toward downtown Salisbury on Sunday. About half an hour later, they were both found collapsed on a bench.

''When she was on the floor, her eyes were just completely white," said Jaime Paine, who found the couple and alerted police. "They were wide open but just white and (she was) frothing at the mouth. Then the man went stiff. His arms stopped moving, but he's still looking dead straight."

A file photo from Facebook is believed to show Yulia Skripal, the daughter of former Russian soldier and spy for Britain, Sergei Skripal, both of whom were left fighting for their lives after suspected poisoning in Salisbury, England, on March 3, 2018. Facebook

For the past 36 hours, police teams in hazmat suits have been scouring the places Skripal had been in Salisbury, including an Italian restaurant and a pub. What they find will determine what happens next.

"The critical thing is to get to the bottom of what's caused this illness as quickly as possible," said Mark Rowley, head of U.K. counter terrorism police. "As you'd expect, the special resources that work within the counter terrorism network that I coordinate across the country and other partners are working with Wiltshire police."

Hours later the London Metropolitan Police confirmed that the counter terrorism division was taking over as the lead agency in the investigation, "due to the unusual circumstances" of the case.

Sergei Skripal speaks to his lawyer from behind bars in this picture of a monitor outside a courtroom in Moscow, Russia, Aug. 9, 2006. AP

"It has not been declared a terrorist incident and at this stage we are keeping an open mind as to what happened," the police said in a statement.

Earlier Tuesday Wiltshire police said in a statement that, "a small number of emergency services personnel were assessed immediately after the incident and all but one have been released from hospital." They said there was no suspected danger to the public, but they didn't provide details on the condition of the one emergency worker who remained hospitalized.

In 2004, Skripal was arrested in Russia and sentenced to prison for spying for Britain. In 2010 he was released in a in prisoner exchange – in return for 10 Russian agents who were operating the in U.S., including the infamous Anna Chapman.

Skripal settled in Salisbury in a modest house where he seems to have led a quiet life, until now.

This incident inevitably brings to mind the 2006 murder of another Russian intelligence officer, Alexander Litvinenko, in London. He was poisoned by radioactive polonium in what British intelligence believes was a Russian state-ordered execution.

Russia has said it's ready to help the investigation, if asked.