British police said on Sunday they had found no evidence so far that anyone else was involved in the death of exiled oligarch and Kremlin critic Boris Berezovsky at his mansion outside London.
Police revealed that Berezovsky, 67, was found by one of his employees on the floor of his bathroom at the house in the upmarket town of Ascot on Saturday.
Paramedics went to the house but Berezovsky was dead when they arrived.
Officers trained in detecting chemical, biological, radiological and nuclear material inspected the property after a device carried by the paramedic suggested the presence of a possibly hazardous substance, but they gave it the all clear.
Police said the death remained "unexplained" but their initial investigation suggested that no-one else was involved.
"It would be wrong to speculate on the cause of death until the post mortem has been carried out. We do not have any evidence at this stage to suggest third party involvement," Detective Chief Inspector Kevin Brown of Thames Valley Police said.
Police said an employee of Berezovsky had called the ambulance service on Saturday after he forced open a door of a bathroom which was locked from the inside and discovered the tycoon's body on the floor.
The employee was the only other person in the house at the time that the body was found, police said.
Berezovsky emigrated to Britain in 2000 after falling out with President Vladimir Putin. He was granted political asylum in 2003 and used Britain as a springboard for attacks on Putin.
But friends of the tycoon said he had become depressed and possibly suicidal recently as his wealth diminished and he had become embroiled in a tussle with his girlfriend.
The suddenness of his death will inevitably lead to speculation. Berezovsky survived one assassination attempt in Russia in 1995 in which a bomb decapitated his chauffeur, and openly expressed fears that his life was in danger.
His friend and fellow Kremlin critic Alexander Litvinenko died an agonising death from radioactive poisoning in London in 2006, in what Litvinenko's widow has said was an assassination by Russian agents.
Last year Berezovsky lost a bitter multi-million pound legal battle in a London court with fellow British-based 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 and accused Abramovich of blackmail, breach of trust and breach of contract in an oil deal.
Following his defeat, he was forced to agree to pay Abramovich £35 million ($56 million) in legal costs, although there is speculation that the final bill will be far greater.
The judge in the case described Berezovsky as "an unimpressive, and inherently unreliable, witness".
Berezovsky said he had the impression "that Putin himself wrote this judgement".
A close confidante of former Russian president Boris Yeltsin, Berezovsky was one of a handful of businessmen who became billionaires following the privatisation of Russian state assets in the 1990s.
But he fell out with Yeltsin's successor, Putin, and fled Russia in 2000 just in time to escape arrest on fraud charges.
In London, Berezovsky became one of the Kremlin's most outspoken critics and is believed to have given financial support to a circle of exiled Russian critics that included Litvinenko.
After the news of Berezovsky's death emerged, Kremlin spokesman Dmitry Peskov said the oligarch had written to Putin recently saying he wanted to come home.
"He asked Putin for forgiveness for his mistakes and asked him to obtain the opportunity to return to the motherland," he said.
But associates of Berezovsky on Sunday cast doubt on the existence of the letter.
"I absolutely don't believe in this letter. This is not possible," opposition activist Andrei Sidelnikov told TV Rain.
Forbes' Russian-language website published an interview Berezovsky gave to a journalist, Ilya Zhegulev, in which Berezovsky said his "life no longer makes sense" and that all he wanted to do was return to Russia.
Zhegulev said the informal interview had taken place on Friday.
Berezovsky's private life has also been turbulent in recent years. His divorce with second wife Galina Besharova in 2011 was dubbed one of the costliest in Britain, and there has been a more recent legal wrangle with his partner Elena Gorbunova.
Berezovsky worked as an academic for nearly two decades before taking advantage of the perestroika reforms to make his fortune.
He used his new political connections to expand into the lucrative energy business, and owned 80 percent of the giant oil company Sibneft by the late 1990s.
However, the fast-talking Muscovite with a taste for the high life fell foul of Putin's crackdown on the oligarchs' political independence.