He was a powerful Russian businessman whose falling-out with his government left him self-exiled in England.
He blamed the Kremlin for the death of a former Russian spy who was poisoned by radioactive material.
Now, Boris Berezovsky himself is dead.
Police scouring the scene Sunday have found no sign so far of "third party involvement," a detective said.
But details surrounding Berezovsky's death remain a mystery. Police say they haven't determined the cause.
After a paramedic's radiation detector went off at the scene, investigators declared the area "safe and clear to work in," the Thames Valley Police said in a statement Sunday.
They did not specify what they believe triggered the alert, but emergency officials from the South Central Ambulance Service said the device, which is a regular part of paramedics' kits, can register false alarms.
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.
Like Berezovsky, Litvinenko had fled to England.
In a deathbed statement, Litvinenko blamed Putin, an accusation the Kremlin strongly denied.
The British government has evidence Russia was involved in Litvinenko's death, a British lawyer said in December. Russia did not immediately comment on the claims of evidence of its involvement in Litvinenko's death.
For years, Berezovsky bankrolled the effort of Litvinenko's widow to push for an inquest into her husband's death.
Berezovsky later sued a Russian broadcaster for libel after it claimed in a report that he was behind the death of Litvinenko.
He won the claim against All-Russian State Television and Radio Broadcasting Company, and the High Court in London awarded him 150,000 pounds ($223,400) in damages.
From math professor to oligarch
Berezovsky began his working life as a math professor and then became a systems analyst who switched to more lucrative jobs in post-Soviet Russia, said CNN's Jill Dougherty, who interviewed him many times.
Berezovsky went on to sell cars "at a time when that was a luxury," she said.