The two sites, the Parkes Hotel and 1 Cavendish Place, were being monitored, the Health Protection Agency said. A third site, the Best Western Hotel on Shaftesbury Avenue, was cleared.
Investigators have now tested for radiation at more than a dozen sites, including three British Airways jetliners and Arsenal soccer club's Emirates Stadium.
As of midnight Sunday, a special medical hot line set up to answer questions had received 3,063 calls since Nov. 25. Of those, 179 people were referred for further follow-up, the agency said.
British police have officially requested assistance from their Russian counterparts and are soon due in Moscow to collect information, prosecutors said Monday.
Alexander Litvinenko, 43, died Nov. 23 in London after being exposed to a rare radioactive element.
The Prosecutor General's office said in a statement they it has to help Scotland Yard officers in their probe. CBS News correspondent Richard Roth reports that Russia issued visas for the team on Monday.
Also Monday, former Russian Prime Minister Yegor Gaidar was released from a Moscow hospital in the evening following a mysterious illness, his spokesman said. Gaidar – who served briefly as prime minister in the 1990s under President Boris Yeltsin and is a leader of a Russian liberal opposition party – began vomiting and fainted during a conference in Ireland on Nov. 24, and was rushed into intensive care at a hospital. He fell ill a day after Litvinenko died in London after being poisoned with the radioactive element polonium-210.
British law enforcement authorities have said a team of nine officers plan to travel to Moscow within days to interview several people, including Andrei Lugovoi, another former spy who met Litvinenko on Nov. 1 — the day he fell ill. FBI agents have also been involved in the probe.
But Roth says the productivity of the British investigators' trip will depend on who they are actually given access to.
"They hope to talk to a variety of people there… outside the SFA, the security apparatus. That's problematic, and we're going to have to take a wait-and-see attitude to see if they are going to be able to meet with the people they want to meet with," terrorism expert Neil Livingstone said on CBS's The Early Show.
Many have speculated that operatives from within the Russian intelligence service, the FSB, or perhaps rogue elements within the agency, are behind the death of their defected colleague.
Yevgenia Albats, an author and journalist who has written extensively on Russia's most secretive agencies, tells Roth there were unwritten but well-defined rules for KGB and now FSB agents to follow.
"If there was a betrayer, somebody who could be considered a betrayer, someone who betrayed the rules of the corporation, the rule was: Sooner or later we will get you, you will get punished," she says.
In a deathbed accusation Litvinenko blamed Russian President Vladimir Putin for the poisoning. The Kremlin has vehemently denied the accusations.
"I think many people are skeptical that Putin himself is very involved in this. This seems to be a very heavy-handed attack," Livingstone told Early Show co-anchor Hannah Storm.
"It's in Putin's interest right now to have a resolution in this case that points at someone other than him," he added.
Britain's senior law and order official said Sunday the inquiry into Litvinenko's death had expanded overseas and a United States-based said he had given police the name of a suspect he believes orchestrated his killing.
"The truth is we have an act of international terrorism on our hands. I happen to believe I know who is behind the death of my friend Sasha (Litvinenko) and the reason for his murder," Yuri Shvets said in an interview with the AP by telephone from the United States.
Shvets, also a former KGB officer, said he had known Litvinenko since 2002 and had spoken to him on Nov. 23, the day he died.
He said he was questioned by Scotland Yard officers and an FBI agent in Washington last week. A police official in London, speaking on condition of anonymity because of the sensitive nature of the case, confirmed officers had interviewed Shvets.
Shvets declined to confirm the name of the person he told police he believed was behind Litvinenko's death, or to offer details of a document he said he had given to the British officers.
"This is first-hand information, this is not gossip. I gave them the first-hand information that I have," Shvets told the AP.
He said he was not prepared to disclose additional details, because of concern he could disrupt the inquiry.
Livingstone, who says he knows Shvets, called him "a reasonably credible individual" and said "there's no reason at this point to think that he is pulling some type of hoax, but, again, he hasn't said publicly who he blames for this, and the authorities aren't saying what type of cooperation they got from him."
Shvets told the AP he had met Litvinenko in 2002, when both men were investigating incidents in the Ukraine.
Livingstone told Storm that an autopsy conducted on Litvinenko's body will likely shed some light on how the former intelligence agent was poisoned.
"We're going to be able to perhaps get a sense of whether it was truly ingested into his stomach and so on, or whether it was, perhaps, inhaled in some way, and therefore various other organs were contaminated differently. So this will give some idea as to how the polonium got into his system," he said.
The autopsy was conducted Friday by medical examiners who took additional precautions to guard against contaminating themselves with the toxic substance. The results have not yet been made public.