"A message has been communicated to anyone who wants to speak out against the Kremlin - 'if you do, no matter who you are, where you are, we will find you and we will silence you - in the most horrible way possible'."
Litvinenko supporter shot in US
Mr Litvinenko died after being
poisoned by a radioactive substance
The FBI and US police are investigating the shooting of a US expert on Russian intelligence, days after he said Moscow had a role in an ex-KGB agent's death.
Paul Joyal, 53, was shot several times as he returned to his home in the suburbs of Washington DC on Thursday.
Reports say Mr Joyal, who was seriously wounded, had items stolen in an attack that appeared to be a random robbery.
But the timing has raised concern that he was targeted for expressing his views on the Alexander Litvinenko case.
Mr Litvinenko, a critic of Russian President Vladimir Putin and the Russian security services, died in November in London after being poisoned by radioactive substance polonium-210.
He and his associates accused Russia of carrying out the poisoning because of his fierce opposition to Mr Putin, who has denied any involvement.
Hospital officials said Mr Joyal was in a critical condition.
'We will silence you'
Four days before he was shot, Mr Joyal told NBC's Dateline television programme that he had struck up a friendship with the former agent during trips to London.
In his interview, Mr Joyal said: "A message has been communicated to anyone who wants to speak out against the Kremlin - 'if you do, no matter who you are, where you are, we will find you and we will silence you - in the most horrible way possible'."
The FBI told the BBC that it was helping the Prince George's County police department in the investigation, but had not opened its own inquiry.
Mr Joyal - a former police officer - runs a consultancy specialising in intelligence information for companies wishing to invest in the former Soviet republics.
Mr Litvinenko was granted asylum in the UK in 2000.