Former Russian spy Alexander Litvinenko died after an apparent poisoning led to a mysterious and rapid decline which left doctors unable to verify his cause of death and led friends on Friday to accuse the Kremlin of murder.
Litvinenko, a vociferous critic of the Russian government, suffered heart failure on Thursday following days of intensive care and extensive medical tests, London's University College Hospital said in a statement.
Doctors discounted earlier diagnoses that the 43-year-old father of three was poisoned with thallium and radiation. Dr. Geoff Bellingan, the hospital's director of critical care, acknowledged he had no clue as to the cause of death.
Anti-terrorist officers from London's Metropolitan police said the matter was "being investigated as an unexplained death," in a statement issued moments after doctors confirmed the ex-intelligence officer had died at 2121 GMT Thursday.
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.
Just hours before he lost consciousness on Tuesday, Litvinenko said he had been silenced by the Kremlin, The Times newspaper quoted a friend as saying.
"The bastards got me, but they won't get everybody," Andrei Nekrasov quoted Litvinenko as saying, according to The Times.
Nekrasov told the AP by telephone from the hospital that Litvinenko's wife Marina, father Walter and his 10-year son Anatoli were by his side when he died.
"I can't use any other word: they whacked another one of us. It was an incredibly professional murder and a sadistic one at that," said Nekrasov.