By Peter Graff
London - British doctors treating an exiled Russian former spy said on Tuesday that they may never know what was used to poison him.
Friends say they believe Alexander Litvinenko, 41, a persistent critic of President Vladimir Putin, was poisoned in a Kremlin-backed plot. Moscow has dismissed the accusations as nonsense.
He is now in a London hospital's intensive care ward - under police guard and fighting for his life.
Doctors earlier said they believed Litvinenko was poisoned with thallium, a highly toxic heavy metal that causes a slow, painful death. But Dr Amit Nathwani said the thallium they detected in his body was not enough to explain his symptoms.
"He has been poisoned so we are looking for other causes of poisoning," Nathwani told reporters outside University College Hospital in London. "But it is also quite possible that we may never find the ultimate cause."
"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 would see in toxicity," he said.
Nathwani said his condition was serious but there had been no major deterioration in the last day.
Britain's anti-terrorism police are investigating the case, which could have far-reaching diplomatic consequences. If Moscow were found to have had a hand in the poisoning it would be the first incident of its kind in the West since the Cold War.
Questioned at a news conference, British Foreign Secretary Margaret Beckett said she had not raised the case with Moscow.
Russia's SVR spy network repeated Moscow's denial of blame.
"There is absolutely no interest for us in occupying ourselves with such activities. Of course everyone has worth, but this person is not worth enough to poison the atmosphere of warm relations between Moscow and London," SVR spokesperson Sergei Ivanov said. "May God give him health."
Earlier, toxicologist John Henry, who is treating Litvinenko, said he now thought the thallium may have been in a more deadly radioactive form or mixed with other chemicals.
Litvinenko has said he fell ill three weeks ago after meeting a source at a London sushi restaurant while studying the slaying of journalist Anna Politkovskaya, another Putin critic gunned down at her Moscow apartment last month.
In Rome, Mario Scaramella, who has helped Italy's parliament investigate Cold War-era Soviet espionage, said he showed Litvinenko an organised crime hit-list bearing his name when they met at the sushi bar.
"I said Alex, I received an alarm in the last few days from a source that you introduced to me," Scaramella said, speaking to reporters in Rome in English.
"He said: 'It's unbelievable. Don't worry about that'."
Henry said injuries to Litvinenko's digestive system suggested he swallowed poison.
He said Litvinenko was very ill and it was too early to say whether he would survive. "At the moment he is not getting better, but he is holding up."
In Moscow, conspiracy theories were swirling.
"I don't rule out that Russian special services had had enough of the talkative Litvinenko," Communist parliamentarian Viktor Ilyukhin told Reuters. But he also said Putin's enemies could be maximising bad publicity to discredit the Kremlin.