(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."

An officer stands outside the nearby Zizzi restaurant, which was closed Tuesday.

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 of London 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 during a Parliament session that the UK would act robustly if "evidence emerges of state responsibility." He then suggested the UK's participation in this summer's World Cup in Russia could potentially be affected.

"I think it will be very difficult to imagine that UK representation at that event could go ahead in the normal way, and we will certainly have to consider that," Johnson said.

A UK Foreign Office representative later clarified those comments, stating that Johnson was referring to the UK's diplomatic representation at the event, rather than its soccer team.

The Russian Embassy in London hit back at Johnson's remarks, saying it "looks like the script of yet another anti-Russian campaign has already been written."

In a statement, the embassy said it was "impressed" by Johnson's comments in Parliament, saying he spoke "in such a manner as if the investigation was already over and Russia was found responsible for what had happened in Salisbury."

"We regret that instead of a proper official clarification on the issue the foreign secretary chose to threaten Russia with retribution," it read.

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 United States as part of the deal was Anna Chapman, who had previously lived in London.

Anna Chapman was among the 10 so-called Russian sleeper agents.

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

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

It quoted Russia's Federal Security Service, or 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 as an intelligence officer during the 1990s and 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.

Police officers stand outside Sergei Skripal's home in Salisbury.

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 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.

Skripal's daughter is thought to be one of the few members of his immediate family still alive after his wife, Lyudmila, and son Alexander died in recent years. Gravestones bearing their names have been seen at a cemetery in Salisbury.