The death of a 69-year-old Russian man in London is being investigated by counter-terrorism police, Scotland Yard has said.
Officers are not naming him because formal identification is yet to take place.
But he is believed to be Nikolai Glushkov – a friend of the late Russian oligarch Boris Beresovsky, who criticised President Vladimir Putin.
After Mr Beresovsky was found hanged at his home near Ascot in Berkshire in March 2013, Mr Glushkov told The Guardian he would “never believe” his friend took his own life.
Mr Glushkov is at the top of a list of “fugitives from justice” published by the Russian embassy.
Mr Berezovsky was also a friend of Russian dissident Alexander Litvinenko, who was poisoned with radioactive polonium-210 in London in 2006.
Police and MI5 are expected to examine allegations that a series of deaths on UK soil, including the Berezovsky case, may be linked to Russia.
Scotland Yard said anti-terror police were investigating Mr Glushkov’s death “as a precaution because of associations that the man is believed to have had”.
But there was “no evidence”, it said, “to suggest a link to the incident in Salisbury”, adding that the death is being treated as “unexplained”.
Sky News’ home affairs correspondent, Mark White, said Mr Glushkov’s home in New Malden, southwest London, is being treated as a crime scene.
Officers were called by the London Ambulance Service at 10.46pm on Monday.
Mr Glushkov’s family are being looked after by specialist police officers.