A former Russian spy and fierce critic of the Kremlin may have been poisoned with radioactive thallium, a doctor said Tuesday. Col. Alexander Litvinenko, an outspoken former KGB and Federal Security Bureau agent, "has some symptoms consistent with thallium poisoning and he's also got symptoms consistent with some other type of poisoning, so it's not a hundred percent thallium," Dr. John Henry, a clinical toxicologist at University College Hospital told reporters. "It could be radioactive thallium," he said. Litvinenko was under armed guard at a London hospital, the victim of what his friends and fellow dissidents called an assassination attempt by the Russian government. "At the moment he's not getting better, but he's holding up," Henry said, adding that Litvinenko was able to eat and to talk. The clinician said it may be too late to determine what poison other than thallium was involved. "It may be that if he has a short half-life of poison, then it's gone. If it's a radioactive poison with a short half-life, it may have gone. Radioactive thallium degrades very rapidly, so that by now we've missed the chance," Henry said. Thallium is widely used in hospitals, "but not in massive doses," he said.