A spokesman for London's University College Hospital says former Russian spy Alexander Litvinenko died while being treated for suspected poisoning.
The spokesman said Litvinenko died late Thursday in an intensive care ward after earlier suffering a heart attack. He said Litvinenko was seriously ill when he was admitted to the hospital on November 17, and the medical team did everything possible to save his life.
Doctors said they were unable to determine the cause of Litvinenko's death. They said tests virtually ruled out poisoning by the heavy metal thallium or radioactive substances that they suspected earlier.
London's Metropolitan Police said they are investigating the circumstances surrounding how Litvinenko became ill. They say they consider the case an unexplained death.
Litvinenko's friends accuse Russian operatives of trying to kill him for his outspoken criticism of President Vladimir Putin. Russia has dismissed the allegation as "nonsense".
Litvinenko said he fell ill November 1, after meeting two Russians at a hotel, followed by lunch at a London sushi bar with Italian academic Mario Scarmella.
Scarmella emerged from hiding on Tuesday. He told reporters in Rome that he met with Litvinenko to confer about a hit list obtained from an unidentified source.
Scarmella said both his and Litvinenko's name were on the list. He alleged that those who drew up the list were also responsible for the unsolved October murder of noted Russian journalist Anna Politkovskaya in Moscow.
Some information for this report was provided by AFP, AP and Reuters.