Here's what we know about the Russian spy critically ill in UK after being exposed to mysterious substance

A former colonel in the Russian military, who passed state secrets to the British, was found slumped on a bench in the U.K. town of Salisbury Sunday after "suspected exposure to a hazardous substance.”

Echoing the 2006 poisoning of former KGB officer Alexander Litvinenko, Sergei Skripal, 66, was found next to a 33-year-old woman, believed to be his daughter Yulya. The pair remain in a critical condition in Salisbury District Hospital.

Specialist counter-terrorism officers were called to help local police investigate, but Mark Rowley, assistant commissioner at the Metropolitan Police, warned against spreading conspiracy theories.

“There are deaths which attract attention,” he told BBC Radio 4’s Today programme Tuesday. “I think we have to remember that Russian exiles are not immortal, they do all die and there can be a tendency for some conspiracy theories. But likewise we have to be alive to the fact of state threats as illustrated by the Litvinenko case.”

However, British Foreign Secretary Boris Johnson warned that should Russian involvement be proven, the U.K. would respond "appropriately and robustly."

Skripal was one of four spies pardoned by Russian President Dmitry Medvedev in 2010 as part of a spy swap deal that saw the repatriation of 10 deep cover sleeper agents planted by Moscow in the U.S. — including “Russia’s most glamorous secret agent” Anna Chapman.

Skripal and the woman, who has yet to be formally named, were found on a bench in the Maltings shopping centre at 4.10 p.m. Sunday. CCTV footage seized by the police from a nearby gym shows the pair walking through an alleyway connecting an Italian restaurant with the bench at 3.47 p.m.

Police closed the restaurant Monday “as a precaution.”

A passerby told the BBC: “They looked like they had been taking something quite strong.”

Skripal worked at the GRU, Russia’s military intelligence agency, and rose to the rank of colonel before moving to the Russian foreign ministry’s office in Moscow. In 2003 he went into business for himself.

However, he was jailed for 13 years by Russia in 2006 for passing the identities of Russian agents working undercover in Europe to MI6.

The FSB said Skripal had been passing state secrets to British intelligence since the 1990s and had been paid $100,000.

Skripal was pardoned in 2010 and given refuge by the U.K. He was assumed to have been given a new identity, but Land Registry records obtained by the Guardian reveal he bought a house in Salisbury under his own name in 2011 for $360,000 with no mortgage.

His wife died two years after he arrived in the U.K. and his son died last year while on holiday in Russia, though the death was not treated as suspicious. Skripal has one surviving daughter who does not live in the U.K.

The pair are “currently being treated for suspected exposure to an unknown substance,” Temporary Assistant Chief Constable Craig Holden told reporters. “The focus is trying to establish what has caused these people to become critically ill.”

“We are working with partners to prioritize this diagnosis and ensure that they receive the most appropriate and timely treatment.”

Dmitry Peskov, Russian President Vladimir Putin's spokesman, told journalists that Moscow had “no information” on the incident, but they were prepared to help with the investigation.

“We see this tragic situation but we don't have information on what could have led to this, what he was engaged in", he said.

One Russian newspaper, Komsomolskaya Pravda, had its own theory as to what happened Skripal:

“It cannot be ruled out that separate groups within British intelligence services are involved in such operations against defectors, who are already entirely in the (control) of new their owners, in order to discredit the Russian Federation and prevent the normalisation of relations between Moscow and London.”

Andrew Foxall, director of the Russia and Eurasia Studies Centre at the Henry Jackson Society, points out that there are now more Russian spies in the U.K. than at the height of the Cold War.

“While it is too soon to attribute responsibility, it would be foolhardy if the authorities were not to explore the Russia connection in relation to Mr. Skripal’s illness,” Foxall said.

Cover image: Pictured in this file image dated August 9, 2006, is retired colonel Sergei Skripal during a hearing at the Moscow District Court. (TASS\TASS via Getty Images)