Specialist police with nuclear and chemical training searched the British home of former Russian oligarch Boris Berezovsky on Sunday, a day after the fervent critic of Russian President Vladimir Putin died in unclear circumstances.
The 67-year-old, a former powerbroker who helped Putin climb to the top of Russian politics before falling from grace, fled to Britain in 2000. His body was found at his house in Ascot, 25 miles west of London.
Police said his death was "unexplained" and sent experts in radioactive, biological and chemical threats to the house as they tried to piece together Berezovsky's final hours.
Berezovsky had survived assassination attempts and said he feared for his life after he became one of Putin's fiercest critics, repeatedly calling for him to be forced from office.
He was also a friend of Alexander Litvinenko, the former Russian spy who was poisoned with radioactive material in London in 2006, a murder that strained diplomatic ties between Britain and Russia.
However, associates said the man who personified the ruthless world of post-Soviet politics may have committed suicide or had a heart attack after the stress of losing a $6 billion court case to Chelsea Football Club owner Roman Abramovich.
Police cordoned off the area around his house and closed roads. Berezovsky's body remained at the scene on Sunday morning, police said.
"It is important we take all necessary measures to ensure a full and thorough investigation can be carried out," said Superintendent Stuart Greenfield, of Thames Valley Police.
"Specially trained officers are currently at the scene, including CBRN (chemical, biological, radiological and nuclear) trained officers, who are conducting a number of searches as a precaution."
Berezovsky - seen by Moscow as a criminal who should stand trial for fraud and tax evasion - was humiliated in 2012 when he lost a legal battle with former partner Abramovich, over shares in Russia's fourth biggest oil company.
Some associates said Berezovsky had struggled with the cost of losing the case, estimated at the time as more than $100 million. Berezovsky had kept a low profile since the defeat and was rarely seen in public.
"My sources say it was heart failure," said Alexei Venediktov, editor of Russia's Ekho Moskvy radio. "After his recent loss in court against Roman Abramovich he was in deep depression."