LONDON - A former Russian spy who said he had been poisoned died Thursday night at a London hospital, following a mysterious and rapid decline that left doctors unable to pinpoint the cause of death, officials said.

This photo released by the family of Alexander Litvinenko shows former Russian security agent Alexander Litvinenko in his hospital bed, at the University College Hospital in central London in this Monday Nov. 20, 2006 file photo. [AP]

Alexander Litvinenko, a fierce critic of the Russian government, had suffered heart failure and was heavily sedated as medical staff struggled to determine what had made the 43-year-old critically ill.

"The matter is being investigated as an unexplained death," London's Metropolitan police said in a statement.

The former spy said he believed he had been poisoned 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 severely damaged, he said.

Just hours before he lost consciousness, Litvinenko said in an interview with The Times newspaper of London that he had been silenced.

"I want to survive, just to show them," he said in the interview published in Friday's edition of the paper, copies of which were available late Thursday. They "got me, but they won't get everybody."

Doctors at London's University College Hospital said tests had virtually ruled out poisoning by thallium and radiation - toxins once considered possible culprits behind the poisoning.

"The medical team at the hospital did everything possible to save his life," hospital spokesman Jim Down said, confirming the Russian's death Thursday night.

"Every avenue was explored to establish the cause of his condition, and the matter is now an ongoing investigation being dealt with by detectives," he said.

Dr. Geoff Bellingan, the hospital's director of critical care, said extensive tests had failed to uncover what had caused Litvinenko to fall ill.

Earlier in the day, hospital officials said Litvinenko was deteriorating rapidly and family members and friends rushed to his bedside.

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