Police investigating the death of Alexander Litvinenko in London have discovered more traces of the radioactive poison that killed him.
They have sealed off the offices of his friend, the tycoon Boris Berezovsky.
What was described as a significant quantity of polonium 210 was found in Mr Litvinenko's urine shortly before his death.
Last night police also sealed off the headquarters of an international security company visited by Mr Litvinenko after finding traces of the substance.
Traces of the substance were earlier detected at the Itsu sushi restaurant in Piccadilly, the Millennium Hotel in Grosvenor Square - both visited by Mr Litvinenko – and at his home in Muswell Hill, in north London.
Four people have now been sent for radiological tests.
Mr Litvinenko was a fierce critic of Russian President Vladimir Putin, and accused Moscow of responsibility for his death.
The Kremlin has denied any involvement.
One area of the intensive care unit at University College Hospital, London, where Mr Litvinenko died, remains closed.
All other areas at the hospitals which had treated Mr Litvinenko are functioning normally.
Crash in France
Scotland Yard declined to comment on whether detectives were investigating possible links to a crash in France in which another Russian billionaire was injured.
Suleiman Kerimov, 32, is in a critical condition in hospital after crashing a Ferrari into a tree on the French Riviera seafront on Saturday.
His passenger, television presenter Tina Kandelaki, 31, also suffered light burns to her legs and was briefly hospitalised before flying back to Russia.
French police are awaiting the results of alcohol and drug tests carried out on the driver and The Sun reported today that Mr Kerimov was to be given radiology tests.
Daily Mirror reporter Graham Brough today became the fourth person to be sent for radiological tests after he tracked down and interviewed Professor Mario Scaramella in hiding in Italy.
Prof Scaramella met Mr Litvinenko the day the former Russian spy fell ill, on 1 November.
He is said to have given Mr Litvinenko a list outlining an alleged plot to assassinate several people, including himself and Mr Litvinenko.
