By Toby Melville
SALISBURY, England (Reuters) - British police raced on Tuesday to identify the substance suspected of striking down a former Russian double agent convicted of treason in Moscow for betraying dozens of spies to British intelligence.
Britain's top counter-terrorism officer, Mark Rowley, said investigators needed to be "alive to the fact of state threats" after Sergei Skripal, once a colonel in Russia's GRU military intelligence service, was taken ill.
The 66-year-old former spy and a 33-year-old woman who was known to him were found on Sunday unconscious on a bench outside a shopping center in the English city of Salisbury after exposure to what police said was an unknown substance.
Both were critically ill in intensive care.
Skripal, who passed the identity of dozens of spies to the MI6 foreign intelligence agency, was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the West as part of a Cold War-style spy swap at Vienna airport.
While the British authorities said there was no known risk to the public, police sealed off the area where the former spy was found and a pizza restaurant called Zizzi in the center of Salisbury. Some investigators wore yellow chemical suits.
"We're speaking to witnesses, we're taking forensic samples at the scene, we're doing toxicology work and that will help us to get to an answer," Rowley told BBC radio. He said counter-terrorism police were assisting the investigation.
"We have to remember: Russian exiles aren't immortal, they do all die and there can be a tendency to conspiracy theories. But likewise we have to be alive to the fact of state threats," he said, pointing to the murder of Alexander Litvinenko.
A British inquiry said President Vladimir Putin probably approved the 2006 murder of ex-KGB agent Litvinenko with radioactive polonium-210 in London. The Kremlin has repeatedly denied any involvement in the killing of Litvinenko.
Litvinenko, 43, an outspoken critic of Putin who fled Russia for Britain six years before he was poisoned, died after drinking green tea laced with the rare and very potent radioactive isotope at London’s Millennium Hotel.
It took some time for British doctors to discern the cause of Litvinenko's illness.
British police did not release the names of those who were being treated but two sources close to the investigation told Reuters that the critically ill man was Skripal. It was unclear what the substance was, they said.
The Kremlin said it was ready to cooperate if Britain asks it for help investigating the incident with Skripal.
"Nobody has approached us with such a request," Kremlin spokesman Dmitry Peskov told a conference call with reporters when asked if the British authorities had been in touch seeking help. "Moscow is always open for cooperation."
Calling it a "tragic situation," Peskov said the Kremlin did not have information about the incident.
Asked to respond to British media speculation that Russia had poisoned Skripal, Peskov said: "It didn't take them long."
Skripal was arrested in 2004 by Russia's Federal Security Service (FSB) on suspicion of betraying dozens of Russian agents to British intelligence. He was sentenced to 13 years in prison in 2006 after a secret trial.
Skripal, who was at the time shown wearing a track suit in a cage in court during the sentencing, had admitted betraying agents to MI6 in return for money, some of it paid into a Spanish bank account, Russian media said at the time.
But he was pardoned in 2010 by then-President Dmitry Medvedev as part of a swap to bring 10 Russian agents held in the United States back to Moscow.
The swap, one of the biggest since the Cold War ended in 1991, took place on the tarmac of Vienna airport where a Russian and a U.S. jet parked side by side before the agents were exchanged.
One of the Russian spies exchanged for Skripal was Anna Chapman. She was one of 10 who tried to blend in to American society in an apparent bid to get close to power brokers and learn secrets. They were arrested by the FBI in 2010.
The returning Russian spies were greeted as heroes in Moscow. Putin, himself a former KGB officer who served in what was then East Germany, sang patriotic songs with them.
Skripal, though, was cast as a traitor by Moscow. He is thought to have done serious damage to Russian spy networks in Britain and Europe.
The GRU spy service, created in 1918 under revolutionary leader Leon Trotsky, is controlled by the military general staff and reports directly to the president. It has spies spread across the world.
Since emerging from the John le Carre world of high espionage and betrayal, Skripal lived modestly in Salisbury and kept out of the spotlight until he was found unconscious on Sunday.
The Sun newspaper said his wife was killed in a car accident shortly after her arrival in Britain. His son was killed in a car accident in Russia.
"On the bench there was a couple, an older guy and a younger girl. She was 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," witness Freya Church told the BBC.
"They looked so out of it that I thought even if I did step in I wasn't sure how I could help, so I just left them. But it looked like they'd been taking something quite strong."
A white and yellow police forensics tent covered the bench where he was taken ill.
(Reporting by Toby Melville; Writing by Guy Faulconbridge; Additional reporting by Alistair Smout, William Schomberg, Andy Bruce and Michael Holden in LONDON, Andrew Osborn and Polina Nikolskaya in MOSCOW and Mark Hosenball in WASHINGTON; Editing by Nick Macfie and Raissa Kasolowsky)