Police investigating the radiation poisoning of Alexander Litvinenko are examining a five-star London hotel.
Radiation was found at an office of Mr Litvinenko's associate
The search has been extended to the Sheraton Park Lane Hotel and a West End office building, Scotland Yard said.
The Health Protection Agency said tests on "key public areas" of the Sheraton found no risk of radiation poisoning.
Eight people have now been referred for tests following Mr Litvinenko's death, the HPA said. More than 1,100 people have called a helpline for advice.
The HPA said 1,121 people had called NHS Direct over the alert by midnight on Monday and 68 cases were followed up.
An HPA spokeswoman said further tests would be carried out at the hotel in any areas identified by the police investigation, she said.
She added the agency would carry out tests at the second new venue, 58 Grosvenor Street, as soon as the police inquiry made it possible.
Mario Scaramella, the academic who met the Russian former spy on the day he was taken ill, has also returned to the UK to be tested. Police will interview him as a potential witness as part of the inquiry into the death.
The HPA said the eight people had been referred to the clinic as a precautionary measure because they had symptoms which may indicate exposure to radiation.
Tests would be aiming to rule this out, it said.
Detectives are continuing to examine five locations in London where traces of polonium-210, also discovered in the body of former KGB colonel Mr Litvinenko, were found.
'No diplomatic barrier'
Meanwhile, Tony Blair said he had not spoken to Russian President Putin over the matter, but would "do so at any time that is appropriate".
"There is no diplomatic or political barrier in the way of (the) investigation going wherever it needs to go," he told a news conference in Copenhagen.
"It obviously is a very serious matter and we are determined to find out what happened and who is responsible."
Camden Council said post-mortem and special examinations of Mr Litvinenko's body would take place on Friday.
Mr Litvinenko was a fierce critic of Russian President Vladimir Putin
The tests have been delayed over concerns about the health implications for those present.
Polonium-210 cannot pass through skin, and must be ingested or inhaled to cause damage.
There is a theoretical risk that anyone who came into contact with the urine, faeces, and possibly even sweat, of Mr Litvinenko could ingest a small amount of polonium-210.
But this kind of radiation has a very short range and can be stopped by a sheet of paper or by the dead layer of outer skin.
The HPA said the risk of the public being exposed remained low and most traces could be eliminated through handwashing, or washing machine and dishwasher cycles.
Contact with carrier's sweat or urine could lead to exposure
But polonium-210 must be ingested to cause damage
Radiation has very short range and cannot pass through skin
Washing eliminates traces
Scotland Yard has been piecing together Mr Litvinenko's movements on 1 November, the day he fell ill.
The first places where polonium-210 radiation was discovered were the Itsu sushi restaurant in Piccadilly, the Millennium Hotel in Grosvenor Square and Mr Litvinenko's home in Muswell Hill, north London.
On the day he was taken ill, he had had meetings at the restaurant with Mr Scaramella and at the hotel's Pine Bar.
Billionaire Russian exile Boris Berezovsky confirmed that his office was another place where traces of radiation had been found during the investigation.
Mr Berezovsky said he was "deeply saddened" by his friend's death.
"I credit him with saving my life and he remained a close friend and ally ever since," he said.
"I will remember him for his bravery, his determination and his honour."
He added he had "complete faith" in the British authorities and the police who were conducting "a thorough and professional investigation".
The fifth location, on the fourth floor of 25 Grosvenor Street, is the headquarters of security and risk management company Erinys.
Erinys said the company "immediately contacted" the police to tell them of a visit made by Mr Litvinenko to its offices.
Mr Litvinenko, 43, who died last Thursday, had been investigating the murder of prominent Russian journalist Anna Politkovskaya.
Both were critics of the Putin government.
Friends have suggested Russian top-level involvement in Mr Litvinenko's death because of his criticisms.
But the Kremlin has repeatedly such allegations as "sheer nonsense".