High levels of radiation have been found in the urine of the former Russian spy Alexander Litvinenko, who died in London last night.
Polonium 210 is now thought to have been responsible for his illness and death.
Britain's Health Protection Agency is now assessing whether anybody who came into contact with Mr Litvinenko has also been contaminated.
Police have also confirmed that they have found traces of radiation at a London sushi bar where the former spy had eaten, and traces of radiation at a hotel.
Meanwhile, Russian President Vladimir Putin has said he was not involved in Mr Litvinenko's death, saying the issue should not evolve into a scandal.
'There is no ground for speculation of this kind,' Mr Putin told a news conference at a summit with the EU.
Finland, which holds the EU presidency, said Mr Litvinenko's death had not been discussed at the summit.
'A death of a man is always a tragedy and I deplore this and send my condolences to the family,' he added.
Earlier Mr Litvinenko's friends said the former agent was killed because of his criticism of Mr Putin.
Mr Litvinenko was a former officer in the KGB and later in the Russian FSB intelligence service.
He had fled to London and believed he was being targeted by the Kremlin for his opposition to Mr Putin's policies.
Initially doctors suspected he had been poisoned with the heavy metal thallium. That was later discounted.
43-year-old Mr Litvinenko had defected to Britain in 2000 and was later granted asylum.
He had been investigating the murder of Russian journalist Anna Politkovskaya, another critic of the Putin government.
In one newspaper today, Mr Litvinenko is quoted as saying shortly before he lost consciousness for the last time: 'They got me but they won't get everybody.'