A Russian exile has been found dead at his home in London as the war of words intensifies following the poisoning of an ex-spy last week.
Nikolai Glushkov, 68, was discovered on Monday evening (local time) by family and friends. The cause of his death is not yet clear.
Metropolitan police say "because of the associations that the man is believed to have had", the counter-terrorism command unit is leading the investigation.
The death is currently being treated as unexplained.
Mr Glushkov is believed to have been close friends with Russian businessman Boris Berezovsky, a reported enemy of Vladimir Putin.
There is no evidence linking his death to the poisoning of ex-spy Sergei Skripal, who remains in a critical condition alongside his daughter after the pair were slipped a suspected nerve agent.
Mr Skripal was once a colonel in Russia's GRU military intelligence service, but had been secretly working for the UK since the mid-1990s. He was arrested by the Russians in 2004, but handed over to the UK in 2010 as part of a spy swap.
British PM Theresa May says it's "highly likely" Russia is behind the poisoning, and has given the country a deadline of Tuesday (UKT) to explain.
A post-mortem for Nikolai Glushkov will be held in the coming days.