The U.S. Federal Bureau of Investigation's experts in weapons of mass destruction will assist with some of the scientific analysis, FBI spokesman Richard Kolko said. Previously, the FBI had only been involved in informal consultations in the case, he said, adding there is no suspected link to the U.S.
In Moscow, meanwhile, doctors said they believed Yegor Gaidar, a former Russian leader and head of a liberal opposition party, may have been deliberately poisoned during a conference last week in Ireland, his spokesman Valery Natarov told The Associated Press.
Gaidar, 50, became violently ill and was rushed to a hospital in Ireland, but was improving in a Moscow hospital Thursday.
He became ill the day after Litvinenko died, but officials have not connected the two cases in any way.
Gaidar, who is a well respected politician and economist in Russia and has been called upon as an adviser to the current president, reportedly received a call in the hospital Friday from President Vladimir Putin, wishing him well.
A coroner in the U.K. formally opened an inquest into the poisoning of Alexander Litvinenko, who died on Nov. 23 after falling ill more than three weeks earlier. It was quickly adjourned so police could continue their investigation, but three pathologists were expected to participate in an autopsy Friday at Royal London Hospital.
Home Secretary John Reid told Parliament that "around 24 venues" have been or are being monitored as part of the investigation, and that experts had confirmed traces of radioactive contamination at "around 12 of these venues." He did not say whether the radioactivity found at the sites was polonium-210.
Reid told lawmakers that officials believed the risk to public health to be low. He said 1,700 calls had been made to the National Health Service, and 69 people were referred to the Health Protection Agency. Of those, 18 who may have been exposed to polonium-210 have been referred to specialist clinics, but all urine tests so far have been negative, he said.
Litvinenko also said before he died that a group of Russian contacts who met with him on Nov. 1 had traveled to London from Moscow, prompting the searches of planes.
Three British Airways planes — two at Heathrow Airport and one in Moscow — are being investigated, and Reid said that a Boeing 737, leased by the Russian airline Transaero, was also "of interest."
Besides that, "there is one other Russian plane that we know of that we think we may be interested in," Reid added. He did not elaborate, except to say that it is Russian.
He said early tests of two of the three British Airways planes showed low levels of a radioactive substance. The third BA plane remains on the ground in Moscow, and has not yet been tested. BA will make a decision whether to bring the plane back from Moscow, he said.
The Transaero jet arrived at Heathrow from Moscow on Thursday, and airline officials said no radioactivity was discovered aboard. "Local security did not find on Transaero planes any toxic substance," said Irena Borodulina, a spokeswoman for Transaero.
The Russian Transport Ministry announced increased radiation checks on international flights and at international airports across the country Thursday.
The three British planes were on the London-Moscow route, but also made stops in Barcelona, Frankfurt and Athens over a period of three weeks. Thousands of passengers aboard some 200 flights have been asked to report any symptoms of radiation poisoning.
It was not immediately clear whether the traces found onboard could have come from passengers who may have come into contact with Litvinenko, or whether a radioactive substance could have been smuggled on board. Authorities refused to specify whether the substance found was polonium-210.
Around 33,000 passengers and 3,000 crew and airport personnel had contact with the 221 flights on the three British planes, said airline spokeswoman Kate Gay. She said the government contacted the airline but would not say what aroused its suspicions.
British Airways has said that "the risk to public health is low," but it has published a list of the flights affected on its Web site and told customers on these flights to contact a special help-line set up by the Health Ministry.
The 43-year-old Litvinenko, a fierce Kremlin critic, had blamed Russian President Vladimir Putin for his poisoning from his deathbed. However, Britain has been careful not to blame the Kremlin for his death, despite criticism of Putin's increasing authoritarianism since the poisoning — even within Prime Minister Tony Blair's Cabinet.
The Russian government has denied any involvement in Litvinenko's death.
Gaidar returned from Ireland to Moscow for treatment earlier this week and was in stable condition on Thursday, said his spokesman, Natarov.
Natarov told the AP that doctors were unable to "detect any natural substance known to them" in Gaidar's body, leading them to believe he may have been poisoned. However, they have not been able to determine what caused specifically his illness and have asked medical experts in Ireland for more information on his condition immediately after he became sick.
Colm Keane, spokesman for National University of Ireland at Maynooth, where the conference was held, said Wednesday that medics initially suspected Gaidar's diabetes or some sort of ailment caused his illness.
A spokesman for the Irish Department of Foreign Affairs, speaking on customary condition of anonymity, said "we have received no evidence of anything untoward about this. He was certainly well enough to travel back home."
Gaidar, an economist, is best known for as the architect of the sweeping free-market reforms that were instituted in the early years of former President Boris Yeltsin's administration. He is one of the leaders of the liberal opposition party Union of Right Forces and heads a think-tank called the Institute for the Economy in Transition.
Maria Gaidar, his daughter, is a well-known liberal youth activist and vociferous Kremlin critic.