There is no evidence so far that a “third party” was involved in the death of Boris Berezovsky, police say.
Earlier, the Russian tycoon’s house in Berkshire was given the all-clear after it was searched by police for chemical, biological and nuclear material.
Thames Police said Mr Berezovsky, 67, was found by an employee dead on his bathroom floor on Saturday afternoon. The door was locked from the inside.
A Home Office post-mortem examination is to be carried out.
Mr Berezovsky emigrated to the UK in 2000 after falling out with Russia’s president, and was granted asylum in 2003.
‘Building a picture’
Police are treating the death as unexplained, while scenes-of-crime officers are currently inside the property carrying out a full forensic examination of the scene.
“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,” Det Ch Insp Kevin Brown of Thames Valley Police said.
“The investigation team are building a picture of the last days of Mr Berezovsky’s life, speaking to close friends and family to gain a better understanding of his state of mind.
“We are acutely aware of the level of interest into his death and are focused on conducting a thorough investigation as we would with any unexplained death.”
Mr Berezovsky’s body was reportedly found by an employee, who called an ambulance at 15:18 GMT on Saturday.
He had not been seen since around 22.30 GMT the previous evening.
His body remained at the property while the search – described by police as a precaution – for chemical, biological, radiological and nuclear material was carried out.
The search was sparked after a paramedic’s personal electronic dosimeter (PED) – a health and safety device – was triggered.
Boris Berezovsky amassed a fortune in the 1990s after the privatisation of state assets following the collapse of Soviet communism.
He survived numerous assassination attempts, including a bomb that decapitated his chauffeur.
In 2003 he was granted political asylum in Britain on the grounds that his life would be in danger in Russia.
He was married twice and had six children – two with each of his wives and two with a long-term partner.
The tycoon’s wealth is thought to have considerably diminished in recent years, leaving him struggling to pay debts in the wake of costly court cases.
In 2011, Mr Berezovsky reportedly lost more than £100m in a divorce settlement. And, last year, he lost a £3bn ($4.7bn) damages claim against Chelsea Football Club owner Roman Abramovich.
In an informal interview with journalist Ilya Zhegule on the eve of his death, and published on Forbes’ Russian language website, Mr Berezovsky reportedly said his life no longer made sense and spoke of his desire to return to Russia.
“There is nothing that I wish more today than to return to Russia,” he is quoted as saying.
“I had underestimated how dear Russia is to me and how little I can stand being an emigre.
“I have changed my opinion on a lot of subjects. I had a very idealistic idea on how to build a democratic Russia. And I had an idealistic idea of what democracy is in the centre of Europe.
“I underestimated the inertia of Russia and greatly overestimated the West.”
On Saturday a Kremlin spokesman said that Mr Berezovsky had recently written to Mr Putin, saying he wanted to go home.
Mr Berezovsky was a close friend of murdered Russian emigre and former KGB officer Alexander Litvinenko, who died in 2006 after he was poisoned with the radioactive material polonium-210 while drinking tea at a London meeting.
Without naming Mr Berezovsky, the Kremlin has accused its foreign-based opponents of organising the assassination. It was thought that Russia was, in part, referring to Mr Berezovsky.
He denied the allegation and accused Mr Putin of personally being behind Mr Litvinenko’s death. A former Russian intelligence officer, Andrei Lugovoi, has refused to attend the Litvinenko inquest, saying he will not receive “justice” in Britain.
Russian media have described Mr Berezovsky’s death as “the end of an era”.
On its website, the pro-Kremlin paper Komsomolskaya Pravda describes Mr Berezovsky as having been “clever, cunning, resourceful… a master of chaos”.
Meanwhile, Novaya Gazeta – which is normally critical of the Kremlin – described him as someone who “viewed Russia as a chess board”, albeit one on which “only he would be allowed to move the pieces”.
Former British ambassador to Russia Sir Andrew Wood, who knew Mr Berezovsky, said he had been a man of vigour who had tended to “over-egg his importance”, was at heart “not a bad man” and had been helpful to Britain in the past.
Sir Andrew told the BBC: “If he killed himself, that’s terrible. If he was poisoned – and it’s interesting that people instantly raise that question in one way or another – that is of course still worse.
“If he had a heart attack, well that was what was coming anyway so one should be grateful if it was fast.” – BBC