As forensic investigators combed the scene Sunday, Berezovsky's body was still inside his house. On Saturday, a bodyguard told police he found Berezovsky lying on the bathroom floor.
Speculation over the tycoon's cause of death ran the gamut from suicide to heart attack.
Some pointed to his declining fortune as a possible clue. Last year Berezovksy lost what has been called one of the most expensive private lawsuits in history.
"Initial suggestions that Berezovsky may have committed suicide were quickly quashed by his close associates," Russia's state-owned RIA Novosti news agency reported.
The agency said a family friend told Russian media that Berezovsky had suffered a heart attack.
"It would be wrong to speculate on the cause of death until the post mortem has been carried out," Kevin Brown, the deputy senior investigating officer in the case, said in a statement.
"The investigation team are building a picture of the last days of Mr. Berezovsky's life," he said, "speaking to close friends and family to gain a better understanding of his state of mind."
A Putin foe
Berezovsky, a staunch critic of Russian President Vladimir Putin, accused the Kremlin of causing the 2006 death of former Russian spy Alexander Litvinenko, who was poisoned with the radioactive substance polonium-210.
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An enemy of Putin found dead you say? I’m shocked... just shocked