London ? His sandy hair has fallen out, and his skin is sallow from the toxin that has attacked his organs and his central nervous system.

Col. Alexander Litvinenko, a former KGB and Federal Security Bureau agent, was under armed guard at a London hospital as authorities investigated the poisoning that has all the hallmarks of a Cold War thriller.

Prominent Russian exiles claimed Litvinenko, now in intensive care, was poisoned at the behest of the Kremlin; Russian authorities denied any link to the attack. Police counterterrorism officials have taken charge of the inquiry.

Doctors said Litvinenko was seriously ill after being given the deadly poison thallium – a toxic metal found in some types of rat poison that can cause damage to the nervous system and organ failure. Such poison has been outlawed in Britain since the 1970s, making it highly unlikely any could have gotten into his food by accident.

Photographs released by the hospital Monday showed a wan Litvinenko in a green hospital gown, his bald head propped up by pillows, his arm hooked to an IV drip. Thallium causes hair loss and interferes with the cardiovascular and nervous systems, attacking the vital organs.

Litvinenko’s white cell count is down to nearly zero, said Dr. John Henry, a clinical toxicologist treating him. “It shows his bone marrow has been attacked and that he is susceptible to infection,” Henry said.

Litvinenko, who has been a thorn in the Russian government’s side since the late 1990s, fell ill after a meal with a contact who claimed to have details about the slaying of another Kremlin critic – Anna Politkovskaya, the Russian investigative journalist who was gunned down Oct. 7 in her Moscow apartment building.

Litvinenko blamed her killing on Russian President Vladimir Putin.

“Somebody has asked me directly, who is guilty of Anna’s death? And I can directly answer you: it is Mr. Putin, president of the Russian Federation,” he said at a meeting at a media club in October in London.

Other Russian dissidents in Britain also blamed the Kremlin for Litvinenko’s condition.

“Permission to assassinate abroad can only be given from the top,” Oleg Gordievsky, former deputy head of the KGB at the Soviet Embassy in London told The Associated Press. “How can it not be state-sponsored?

“He was for five years attacking Putin and the head of the (secret services) week in, week out. He was deliberately irritating the whole of the Russian establishment, particularly Putin.”

In Moscow, Kremlin spokesman Dmitry Peskov dismissed suggestions that Russian intelligence services were involved as “nothing but sheer nonsense.”

But exiled Russian dissident Boris Berezovsky was adamant that Putin, a former spy himself, was not only aware of the operation, but gave the orders.

“There’s no doubt Putin gave the command to kill him,” said the multimillionaire, who claimed that Litvinenko also had been targeted a year ago, when a grenade was thrown into his home.

Alexander Goldfarb, who helped Litvinenko seek asylum in Britain in 2000, said the poison might have been sprinkled into Litvinenko’s drink during a meeting at a London hotel Nov. 1 before he went to meet the contact at a sushi restaurant.

Litvinenko briefly met two men from Moscow – one a former KGB officer he knew – for tea at the hotel, Goldfarb said.

“I called Alexander in hospital … he told me it is true, on that day, before meeting the Italian, he met with two Russians,” Goldfarb said, adding that Litvinenko had not previously met the second man.

Litvinenko told police about the two men, he said.