Officials have asked anyone who visited the London sushi bar or hotel where Litvinenko held meetings on November 1, the day he is believed to have been poisoned, to call authorities so they can possibly tested for the radioactive substance that has been linked to Litvinenko's death.
Investigators say they are trying to determine the source of the rare isotope polonium-210 that was detected in Litvinenko's body.
Traces of the substance have been found in the restaurant and hotel, and officials say it is possible other people may have been contaminated. Officials have stressed, however, that the likelihood that others were exposed remains low unless they had close contact with Litvinenko.
Investigators say they have begun looking for witnesses and reviewing surveillance tapes in a bid to determine how Litvinenko may have ingested the polonium-210.
(compiled from agency reports)