The FBI has joined the poisoned spy probe that's gripped governments from London to Moscow the agency announced Thursday as investigators found traces of radiation at several sites in Britain.

British authorities requested the involvement of the FBI, agency spokesman Richard Kolko said. FBI experts in weapons of mass destruction will assist with some of the scientific analysis, he said.

Meanwhile, the story took another bizarre twist when aides to former Russian Prime Minister Yegor Gaidar claimed that he, too, was intentionally poisoned.

The revelation came as British officials announced they had identified 12 sites showing traces of radiation and were expanding their investigation of flights between London and Moscow to include Russian aircraft.

Gaidar, Russia's first post-Soviet premier under Boris Yeltsin, fell ill at breakfast last Friday in Dublin, Ireland, just a day after former KGB agent Alexander Litvinenko died from radiation poisoning.

"I went up to him. He was lying on the floor unconscious. There was blood coming from his nose, he was vomiting blood. This went on for more than half an hour," his daughter Maria said.

The 50-year-old former prime minister, who has been a mild critic of current Russian President Vladimir Putin, was rushed to a local hospital where he remained unconscious for three hours.

One of Gaidar's aides, Valery Natarov, said that doctors treating him at a Moscow hospital, where he appears to be making a recovery, believe he was deliberately poisoned.

"Doctors don’t see a natural reason for the poisoning and they have not been able to detect any natural substance known to them," Natarov told the Times of London. "So obviously we’re talking about poisoning [and] it was not natural poisoning."

The investigation, meanwhile, into the poisoning death of ex-KGB agent Litvinenko found traces of radiation on two British Airways planes and at a dozen London sites; three more aircraft were being tested, including a Russian commercial jet. In addition, some 33,000 airline passengers and 3,000 crew and airport personnel were being sought for radiation testing.

A coroner formally opened an inquest into Litvinenko’s poisoning after he suffered for three weeks before dying on Nov. 23. 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.

A former spy who met with Litvinenko on the day he was allegedly poisoned, Andrei Lugovoy, is a former KGB colleague who once served as Gaidar's bodyguard.

Gaidar is one of the leaders of a liberal opposition party who served briefly as prime minister in the 1990s under President Boris Yeltsin. Gaidar was in Ireland to promote a new book.

The Kremlin announced Friday that Putin had telephoned Gaidar to wish him "a speedy recovery."

International Atomic Energy Agency spokeswoman Melissa Fleming said Britain has not asked the U.N. watchdog agency for help in tracing the radioactive substance, "but we stand ready to assist, and we communicated that."

Anatoly Chubais, a top Yeltsin-era government official and now head of the national electricity monopoly, said Wednesday that he suspected a link among Gaidar's illness, Litvinenko's death and last month's murder of investigative reporter Anna Politkovskaya.

On the day he first felt ill, Litvinenko said he had two meetings. In the morning, he met with an unidentified Russian and with Lugovoy at a London hotel. Later, he dined with Italian security expert Mario Scaramella to discuss the October murder of Politkovskaya.

Scaramella told reporters in Rome on Tuesday that he had traveled to meet Litvinenko to discuss an e-mail he received from a source naming the killers of Politkovskaya, who was gunned down Oct. 7 at her Moscow apartment building, and outlining that he and Litvinenko were on a hit list.

High doses of polonium-210, the type of radiation found in Litvinenko’s corpse, is a rare radioactive element usually made in specialized nuclear facilities. The investigation is focusing on places and people with which the former spy had contact in the weeks before he became sick.

British 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. "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.

From his deathbed, the 43-year-old Litvinenko, a fierce Kremlin critic, had blamed Russian President Vladimir Putin for his poisoning. He told police he believed he had been targeted for investigating the October killing of Russian journalist Politkovskaya, another critic of Putin's government who was gunned down in her Moscow apartment building.

The Associated Press, Times of London and Financial Times contributed to this report.