British police said on Thursday that they were now treating their investigation into the death of former Russian spy Alexander Litvinenko as a murder inquiry.
Litvinenko fell ill on November 1 and died on November 23 in a central London hospital. Unexplained large amounts of the radioactive isotope polonium 210 were discovered in his urine.
The death has triggered a large-scale international investigation, meetings of the British government's top security cabinet and appeals for Russia to provide assistance to the probe.
"Detectives investigating the death of Alexander Litvinenko have reached the stage where it is felt appropriate to treat it as an allegation of murder," London's Metropolitan Police said in a statement.
"Detectives from the Metropolitan Police Service Counter Terrorism Command are pursuing many lines of inquiry, both in the UK and Russia, and have spoken to a number of witnesses in connection with the death."
It cautioned, however, that detectives were keeping an open mind and were methodically following the evidence.
"It is important to stress that we have reached no conclusions as to the means employed, the motive or the identity of those who might be responsible for Mr Litvinenko's death," the statement said.
It added that the inquiry was still in its early stages and that the police were not prepared to comment on "speculative reports" in the media.