Moscow ? Scotland Yard investigators looking into the mysterious poisoning of former KGB spy Alexander Litvinenko took their probe Monday to the Russian capital, where Russian leaders pledged cooperation but expressed growing frustration with accusations of Kremlin involvement in Litvinenko’s death.

The arrival of a team of British detectives in Moscow significantly broadens the investigation into the Nov. 23 death of Litvinenko, who was poisoned with polonium-210, a rare radioactive isotope.

Traces of radiation were found on British Airways planes that had flown the Moscow-London route, as well as in Litvinenko’s home and other locations in London. Also, three businessmen whom Litvinenko met Nov. 1, the day he fell ill, were Russians who came to London from Moscow and sat down with Litvinenko in a London hotel where radiation traces also were discovered.

British news media have reported that Scotland Yard investigators want to interview the businessmen, Dmitry Kovtun, Vyacheslav Sokolenko and Andre Lugovoi, a former colleague of Litvinenko’s in the KGB. Lugovoi has told Russian media that he and the other two men who met Litvinenko are innocent and are being framed by Litvinenko’s real killers.