A London hospital confirmed late Thursday that the former Russian spy who alleged he'd been poisoned has died.

In a photo released by his family, former Russian security agent Alexander Litvinenko as he appeared in his hospital bed at University College Hospital on Monday.
(Family handout/Associated Press)

Alexander Litvinenko died at 9:21 p.m. local time at University College Hospital. A spokesperson for the hospital said that New Scotland Yard would be investigating the circumstances of his death.

Litvinenko's condition deteriorated dramatically Thursday as heart failure left him critically ill and on life support as family members rushed to his bedside.

Litvinenko, a fierce critic of the Russian government, was under heavy sedation in a London hospital as doctors struggled to determine what sickened the 43-year-old, a family friend said.

Doctors virtually ruled out thallium and radiation — toxins once considered possible culprits behind the apparent poisoning of Litvinenko.

"Despite extensive tests, we are still unclear as to the cause of his condition," said Dr. Geoff Bellingan, director of critical care.

Alex Goldfarb said Thursday morning that his friend Alexander Litvinenko's condition had deteriorated and he suffered heart failure overnight.
(CBC)

Litvinenko believed he was given poison on Nov. 1 while investigating the slaying of another Kremlin detractor — investigative journalist Anna Politkovskaya.

His hair fell out, his throat became swollen and his immune and nervous systems were damaged. A friend, Andrei Nekrasov, said Litvinenko's skin had turned yellow, a possible effect of liver failure.

Another family friend, Alex Goldfarb, joined Litvinenko's wife Marina, his son Anatoli and the former agent's father by his bedside earlier in the day.

"He went into a cardiac failure overnight and the hospital put him on artificial heart support," Goldfarb said. "He's on the ventilator, he's getting artificial resuscitation."

Police investigate poisoning allegations

Anti-terrorist police were investigating the alleged poisoning, which friends and dissidents allege was carried out at the behest of the Russian government. Litvinenko sought asylum in Britain in 2000, and has been a relentless critic of the Kremlin and the Russian security services ever since.

On Wednesday, the Russian Foreign Intelligence Service, the SVR, issued its strongest denial yet that it was involved in any assassination attempt.

"Litvinenko is not the kind of person for whose sake we would spoil bilateral relations," SVR spokesman Sergei Ivanov said, according to the Interfax news agency. "It is absolutely not in our interests to be engaged in such activity."

Litvinenko worked both for the KGB and for a successor, the Federal Security Service. In 1998, he publicly accused his superiors of ordering him to kill Russian tycoon Boris Berezovsky, who is exiled in Britain.

A year later, Litvinenko spent nine months in jail on charges of abuse of office, a charge on which he was later acquitted. That experience prompted his move to London.

Two meetings on day symptoms appeared 

On the day he first felt ill, Litvinenko said he had two meetings. In the morning, he met with an unidentified Russian and with Andrei Lugovoy, a former KGB colleague and bodyguard to one-time Russian Prime Minister Yegor Gaidar at a London hotel.

Later, he dined with Italian security expert Mario Scaramella to discuss the October murder of Politkovskaya.

Scaramella told reporters in Rome on Tuesday that he had travelled to meet Litvinenko to discuss an e-mail he received from a source naming the killers of Politkovskaya, who was gunned down Oct. 7 at her Moscow apartment building, and outlining that he and Litvinenko were on a hit list.

Goldfarb said that he had a photocopy of the four-page e-mail and confirmed that it did read like the hit list described by Scaramella.

But he refused to say who compiled the document, saying that it could jeopardize the police investigation into the poisoning.

After visiting the hospital on Thursday, Berezovsky told the Associated Press that British police have yet to speak to him, but hoped they would be in contact over the next two days.