Police sealed off the British home of exiled Russian oligarch and Kremlin critic Boris Berezovsky on Sunday and sent in hazardous material experts following the discovery of his body at the house.
The 67-year-old was found dead in his mansion in the well-heeled commuter town of Ascot, near London, on Saturday afternoon.
His lawyer Alexander Dobrovinsky said his death may have been suicide brought on by depression over his debts.
But since the tycoon survived one assassination attempt in 1995 and remained fearful of other bids to kill him, his death is bound to provoke speculation.
Police shut down the streets surrounding the gated property overnight and sent in chemical, biological, radiological and nuclear (CBRN) experts to investigate what they described as an "unexplained" death.
Berezovsky's friend and fellow Kremlin critic Alexander Litvinenko was killed by radioactive poisoning in London in 2006, in what his widow has said was an assassination by Russian agents.
"Specially trained officers are currently at the scene, including CBRN trained officers, who are conducting a number of searches as a precaution," a statement from Thames Valley Police said.
They were present to enable police officers to carry out their work, the statement said. Almost 10 hours after Berezovsky died, his body had still not yet been removed, it added.
Police Superintendent Stuart Greenfield said: "I would like to reassure residents that we are confident there is no risk to the wider community."
Berezovsky was one of handful of businessmen who became billionaires following the privatisation of Russian state assets in the 1990s, but his fortunes had slumped in recent years.
He was a confidante of former president Boris Yeltsin but fell out with his successor, President Vladimir Putin, fleeing Russia in 2000 just in time to escape arrest on fraud charges.
In London, Berezovsky became one of the Kremlin's most outspoken critics, leading a circle of exiled Russian critics that had included Litvinenko before his agonising death.
Paramedics were called to Berezovsky's estate at 3:18 pm (1518 GMT) on Saturday and the Russian was pronounced dead at the scene, the ambulance service said.
"His body was found by his bodyguard," said a spokesman for Berezovsky, refusing to comment on media reports that he had been found in his bath.
In 1995, Berezovsky narrowly escaped an attempt on his life in which his driver was decapitated, and he remained fearful of other attacks.
His lawyer however told Russian state television that he had been informed by contacts in London that Berezovsky had killed himself.
"Berezovsky has been in a terrible state as of late. He was in debt. He felt destroyed," said Dobrovinsky. "He was forced to sell his paintings and other things."
However, the oligarch's friend Demyan Kudryavtsev firmly denied that Berezovsky had killed himself.
"There are no external signs of a suicide," he was quoted as saying by the Prime news agency in Russia.
"There are no signs that he injected himself or swallowed any pills. No one knows why his heart stopped."
Last year, the tycoon lost a bitter multi-million pound legal battle with Russian fellow oligarch Roman Abramovich, the owner of Chelsea Football Club.
Berezovsky had sought more than £3 billion ($4.75 billion, 3.8 billion euros) in damages, accusing Abramovich of blackmail, breach of trust and breach of contract in an oil deal.
When he lost, he agreed to pay Abramovich £35 million ($56 million) in legal costs, although there is speculation that the full fees would come to far more than that.
Berezovsky's private life has also taken its toll. A 2011 divorce with his second wife Galina Besharova was dubbed the costliest in Britain, and there has been a more recent legal wrangle with his partner Elena Gorbunova.
Born on January 23, 1946, in Moscow, Berezovsky worked as an academic for nearly two decades before taking advantage of the peristroika reforms to make his fortune.
However, the fast-talking Muscovite with a taste for the high life fell foul of Putin's crackdown on the oligarchs' political independence. In 2003, Britain granted him political asylum.
After news of Berezovsky's death, Kremlin spokesman Dmitry Peskov said the oligarch had written to Putin a couple of months ago saying he wanted to go home.
"He asked Putin for forgiveness for his mistakes and asked him to obtain the opportunity to return to the motherland," he told Russian state television.
Forbes' Russian-language website published an interview he gave to journalist Ilya Zhegulev, in which he said his "life no longer makes sense" and that there was nothing he wanted more than to return to Russia.
Zhegulev said it had been an informal interview given on Friday evening, which he had not recorded. While he had promised Berezovsky not to publish it he had changed his mind after hearing of his death.