Russian prosecutors have asked the UK for permission to question more than 100 witnesses over the poisoning of Russian ex-spy Alexander Litvinenko.
Alexander Litvinenko was a fierce critic of President Putin
Russian investigators also want to examine "dozens" of places in Britain in connection with Litvinenko's death, the deputy chief prosecutor said.
Alexander Zvyagintsev said Russia had "many questions" concerning the case.
Mr Litvinenko, 43, died in a London hospital on 23 November. His body contained radioactive polonium-210.
British detectives visited Moscow in December to question witnesses as part of their inquiry.
Mr Zvyagintsev said Russia had provided the information they requested quickly and that translators had worked round-the-clock to help Scotland Yard.
His comments were published in the Russian government newspaper Rossiiskaya Gazeta on Friday.
Friends of Mr Litvinenko believe he was poisoned by the Russian government because of his strong criticism of President Vladimir Putin. The Kremlin has dismissed suggestions it was involved in any way.
Mr Litvinenko, a former Federal Security Service (FSB) officer, met three other former Russian secret agents just before he fell ill in London - Dmitry Kovtun, Andrei Lugovoi and Vyacheslav Sokolenko. They denied any involvement in his death.
The UK's Health Protection Agency (HPA) says 120 of the 596 people tested for polonium-210 have tested positive for contamination.
But the HPA said just 13 were deemed to have a health risk.
The HPA's chief executive, Professor Pat Troop, said tests were still being carried out on a number of foreign nationals who may have been contaminated.
Mr Litvinenko visited a number of central London venues on the day that he fell ill.
The Millennium Hotel in Mayfair, the Itsu sushi bar in Piccadilly and an Italian restaurant, also in Mayfair, were among the addresses.
Prof Troop said the amount of contamination in the former spy's body was "many thousands of times greater" than anyone else who had tested positive for polonium-210.