The unconscious woman found with a Russian spy is his daughter, a source says

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

British counter-terrorism police are investigating how the pair ended up slumped on a bench in the southern English city of Salisbury.

The incident has not been declared terrorism. But due to the unusual circumstances, the specialized counter-terrorism unit will lead the investigation, London's Metropolitan Police said in a statement Tuesday.

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

Salisbury, perhaps best known as the stopping-off point for tourists visiting nearby Stonehenge, has since 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 British 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.

Images taken outside the restaurant on 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 after the incident, local police said in a statement Tuesday. One of them remains in the hospital.

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."

By Tuesday morning, the bench had been covered in a white forensic tent, a light drizzle of rain falling on reporters gathered at the scene.

The case has drawn comparisons to the poisoning of former Russian spy Alexander Litvinenko, who died a slow death after drinking tea laced with highly radioactive polonium-210 in a Mayfair hotel in 2006.

A detailed UK inquiry later concluded that Russian President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko. The Russian Foreign Ministry dismissed the UK investigation as politically motivated.

In a conference call with reporters Tuesday, Kremlin spokesman Dmitry Peskov declined to comment on the Skripal case, saying "we do not have any information" about the situation, and adding that he did not know whether Skripal still had Russian citizenship.

UK Foreign Secretary Boris Johnson said Tuesday the Skripal case had "echoes" of what happened to Litvinenko.

While he stopped short of "pointing fingers" of responsibility, Johnson said the UK would act robustly if "evidence emerges of state responsibility," during a Parliament session.

Who is Sergei Skripal?

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 10 so-called Russian "sleeper agents" deported by the US as part of the deal was Anna Chapman, who had previously lived in London.

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

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

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.

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 undercover in Europe, 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.