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LONDON, England (CNN) -- Doctors are still trying to determine how a former Russian spy and prominent Kremlin critic was poisoned, but they have discounted suspicions that the toxic metal thallium was used, hospital officials said Tuesday.
Alexander Litvinenko remained in serious condition at University College Hospital in London, where he has been treated since Friday.
He says he was poisoned after meeting with an Italian who claimed to have information connecting the Russian government with the October slaying of a frequent critic, journalist Anna Politkovskaya.
He met his contact at a London sushi restaurant in early November and fell violently ill within hours. Doctors say he is at risk of heart and kidney failure and may need a bone marrow transplant if he survives.
His sudden illness has led Litvinenko and his friends to suggest that Putin's government was behind the poisoning, an allegation Moscow denies.
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 has suffered from dehydration and heart complications, and his hair has fallen out. That combination of symptoms led Dr. Paul Henry, a clinical toxicologist who has examined Litvinenko, to say Monday that the cause may have been radioactive thallium -- a heavy metal sometimes used in cardiac tests.
Henry called thallium -- which looks like sugar or salt, dissolves in water and has no taste or odor -- "an ideal homicidal poison." But Dr. Amit Nathwani, a consulting physician on Litvinenko's case, said Tuesday that doctors have found other evidence steering them away from thallium.
"His symptoms are slightly odd for thallium poisoning, and the levels of thallium we were able to detect are not the kind of levels you'd see in toxicity," Nathwani said. He said the substance could not be ruled out completely, "but it is also quite possible that we may never find the ultimate cause."
Litvinenko's treatment is likely to take "weeks and months," the doctor said.
The ex-spy 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.
Scotland Yard is investigating his poisoning, and Litvinenko has spent more than 15 hours talking with detectives during his hospitalization, a friend, Alex Goldfarb, said.
"He's exhausted, so he's resting," Goldfarb said.
Moscow recently extended government powers to deal with "extremists," he said -- "So all of this is legal under Russian law." But others say Litvinenko had underworld connections that might have been behind his poisoning.
"He was working amongst terrorists, against gangsters, so he lived in that world," said Ivor Gaber, a political analyst at London's Goldsmith University. "He was very aware of the enemies he made in his professional life, but of course, he was now making an enemy of the Russian state."
CNN Correspondents Paula Newton and Matthew Chance contributed to this report.