London ? Alexander Litvinenko, a former Russian spy and vocal critic of Russian President Vladimir Putin, died Thursday night in a London hospital after being mysteriously poisoned.

Litvinenko, 43, was placed under armed guard at a London hospital last week after medical tests confirmed he had been poisoned. Doctors said his head of thick, brown hair suddenly fell out and he suffered severe liver and bone marrow damage. But they said they have been unable to identify what caused the exceptionally fit man to start vomiting Nov. 1, grow steadily sicker and die.

Litvinenko was a colonel in the Federal Security Service, FSB, the domestic successor to the KGB, who became highly critical of some of his superiors. He is well known for accusing FSB agents of involvement in apartment building bombings in 1999 that killed more than 300 people. Russian officials blamed the attacks on separatists from Chechnya and launched a new military offensive in the republic.

Recently, Litvinenko began looking into the unsolved killing of investigative journalist Anna Politkovskaya, who was shot last month in Moscow. She too was a prominent critic of Putin.