LONDON — The FBI is joining the British probe into the poisoning death of a Kremlin critic, the agency announced yesterday as investigators found traces of radiation at a dozen sites in Britain and a former Russian prime minister reported symptoms consistent with poisoning.

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.

There is no suspected link to America in an investigation that extends to five airliners and locations from London to Moscow. Russian officials said radiation levels were normal on two suspect Russian jets and appealed to British officials for information on how to test Russians who traveled aboard the two British Airways planes on which radiation has so far been detected.

Yegor Gaidar, who served briefly as prime minister in the 1990s under President Yeltsin of Russia, vomited and then fainted during a conference in Ireland on Noember 24, a day after ex-KGB agent Alexander Litvinenko died of radiation poisoning. Doctors treating Mr. Gaidar in Moscow believe that he was also poisoned, a spokesman, Valery Natarov, said.

While Litvinenko was a fierce critic of the Kremlin who during his waning hours blamed President Putin for his poisoning, Mr. Gaidar, who is one of the leaders of a liberal opposition party, is a figure with little influence in today’s Russia whose moderate criticism of the Kremlin has focused on economic issues.