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LONDON, England (CNN) -- A Russian former spy died Thursday night in a London hospital three weeks after his suspected poisoning, with doctors unable to determine the cause of his illness, hospital officials and police said Thursday.
Alexander Litvinenko was a longtime critic of the Russian government, which he and his friends blamed for his sudden illness earlier this month. Russian authorities have denied any role in the matter.
Doctors reported earlier Thursday that his condition had suffered a "major deterioration" overnight, and that extensive tests had failed to turn up the cause of his illness. He was pronounced dead at 9:21 p.m. Thursday (4:21 p.m. ET), hospital spokesman Jim Down said.
"Inquiries continue into the circumstances surrounding how Mr. Litvinenko, 43 years, of north London, became unwell," Scotland Yard reported.
Down said doctors at University College Hospital "did everything possible to save his life." He declined further comment, citing the ongoing investigation.
"Our thoughts are with Mr. Litvinenko's family," he said.
A friend, Alex Goldfarb, said Wednesday that Litvinenko had suffered heart failure and was placed on life support.
Litvinenko said he was poisoned after meeting with a contact who claimed to have information connecting the Russian government with the October slaying of a frequent critic, journalist Anna Politkovskaya.
Litvinenko fell ill hours after the meeting, which took place at a London sushi restaurant -- but was able to spend more than 15 hours talking with detectives during his hospitalization, Goldfarb said.
His combination of symptoms -- including dehydration, heart complications and hair loss -- led doctors to suspect the heavy metal thallium. Tests ruled out thallium poisoning or any radioactive material, and one of his doctors, Amit Nathwani, said it was "quite possible" that the cause would never be known.
"The matter is being investigated as an unexplained death," London's Metropolitan police said in a statement carried by The Associated Press.
Litvinenko was once a colonel in Russia's Federal Security Service, the FSB. He has been a defender of the Chechen separatists who have battled Moscow's rule for much of the past 15 years, and has accused the government of orchestrating the bombings of a string of apartment buildings as a pretext for its 1999 invasion of the breakaway republic.
He left Russia in 2000, accusing his former agency of planning to kill opponents of Russian President Vladimir Putin, and he recently blamed the Kremlin for Politkovskaya's death.
The Kremlin has denied any role in Litvinenko's death. FSB spokesman Sergei Ivanov told the Russian news agency Interfax on Wednesday that the agency was "sorry for what has happened to him," and wished Litvinenko a "speedy recovery."
Ivanov suggested the culprit lay among Litvinenko's associates in London. And others say Litvinenko had underworld connections that might have been behind his poisoning.
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