British police are treating the death of ex-Russian spy Alexander Litvinenko as murder, they have confirmed.
Mr Litvinenko died on 23 November in a London hospital
The news comes as small traces of a radioactive substance were found at the British embassy in Moscow following a precautionary check.
But officials said the levels of radiation found would not pose a risk to public health.
The former KGB agent's death on 23 November, in London, has been linked to the highly toxic isotope polonium-210.
"Detectives in this case are keeping an open mind and methodically following the evidence," Scotland Yard said in a statement.
"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 British embassy announced on 4 December it would test one of its rooms as a precaution, after former KGB bodyguard Andrei Lugovoi visited the building to deny any involvement in the poisoning of Mr Litvinenko, 43.
Mr Lugovoi and another Russian businessman reportedly met with Mr Litvinenko in London on 1 November, the day he fell ill.
Tests have been carried out at a number of venues Mr Litvinenko visited in London on that day.
Earlier, Mario Scaramella, the Italian academic who also met Mr Litvinenko on 1 November - at a sushi bar - was discharged from hospital in London.
Mr Scaramella was under observation after testing positive for polonium-210.
In an interview from his hospital bed he told CNN he had travelled to London for a conference, but had changed his plans to meet Mr Litvinenko.
He said had received e-mails claiming that both he and Mr Litvinenko were "under the special attention of hostile people", but neither man believed the threats were real.
Friends believe Mr Litvinenko was poisoned because of his criticisms of the Russian government, but the Kremlin has dismissed suggestions it was involved in any way as "sheer nonsense".
Meanwhile, Dmitry Kovtun, a business associate of Mr Lugovoi, is reported to have been interviewed by British police in Moscow.
The pair met Mr Litvinenko on 1 November at the Millennium Hotel.
Mr Lugovoi is also expected to be interviewed in Moscow.
Russian officials are expected to conduct the interview but British detectives will be in attendance.
Nine Metropolitan police officers are currently in Moscow but have had restrictions placed on their investigations into Mr Litvinenko's death by the authorities.
Russia's chief prosecutor, Yuri Chaika, said British officers could not arrest Russian citizens and suspects would not be extradited to Britain.