The family of former Russian Prime Minister Yegor Gaidar - who collapsed during a visit to Ireland last Friday - believes he was poisoned.
Mr Gaidar, who had been treated in Connolly Hospital in Dublin after becoming ill during a conference in NUI Maynooth, is now being treated for a mystery illness in a Moscow clinic.
Gardaí say they are investigating the matter but that there's no need for public alarm.
The Kremlin has said President Vladimir Putin telephoned Mr Gaidar at the hospital in Moscow yesterday and wished him a speedy recovery.
Russian doctors said they have identified no natural cause for an ailment afflicting Mr Gaidar, according to his spokesperson.
'According to preliminary information from doctors, they do not see at the moment a natural reason for the poisoning,' Valery Natarov told Reuters.
'They think it is a substance they cannot so far identify - it is not a natural poisoning,' he said. 'It is too early to say whether it is a poison or not.'
His illness followed the death in London of the former spy Alexander Litvinenko, who was poisoned with a radioactive substance.
An inquest in London into Mr Litvinenko’s death has been adjourned and a post mortem is to take place tomorrow.
The Kremlin has denied any link to the death of Mr Litvinenko, who accused Mr Putin of killing him.