Russia's chief prosecutor says he is investigating whether a former boss of the oil firm Yukos may be linked to the death of ex-agent Alexander Litvinenko.
Alexander Litvinenko suspected the Kremlin of poisoning him
The prosecutor-general's office said it was "checking the version" that former Yukos manager Leonid Nevzlin could be among those involved in the poisoning.
Mr Nevzlin's spokesman said the suggestion was "ridiculous", Reuters news agency reported.
Mr Nevzlin was close to jailed tycoon Mikhail Khodorkovsky, who ran Yukos.
The prosecutor-general's statement said Mr Nevzlin and some other ex-Yukos executives were wanted internationally "over serious crimes".
It said there were indications of a link between the poisoning of Mr Litvinenko and attempted murder of Dmitry Kovtun - who met him in London before he fell ill - and "the charges that several Yukos managers committed crimes against the life and health of citizens".
Demise of Yukos
Mr Nevzlin, who denies any wrongdoing in connection with Yukos, is now living in Israel.
The prosecutor-general's office says it is taking steps towards the possible extradition of the Russian suspects living abroad.
Mr Nevzlin fled to Israel after prosecutors acted against Yukos
Former secret service officer Mr Litvinenko died in London on 23 November and his body was found to contain a massive dose of the radioactive isotope polonium-210.
The case - which remains shrouded in mystery - has strained relations between Britain and Russia.
A statement made by Mr Litvinenko before he died accused Russian President Vladimir Putin of involvement in his death - but the Kremlin has dismissed any suggestion it was involved as "nonsense".
Mr Khodorkovsky is facing new money-laundering charges along with his jailed colleague Platon Lebedev.
Most of the enormous assets of Yukos have been taken over by the state-controlled oil company, Rosneft.