Chemical and biological experts in Britain are investigating the unexplained death of the exiled Russian tycoon Boris Berezovsky.
The circumstances of the death of the 67-year-old - a wanted man in Russia and opponent of President Vladimir Putin - are not yet known and police are treating the death as unexplained.
Police with expertise in environments contaminated with chemical, biological and nuclear material are searching his Berkshire house where he was found dead on Saturday.
A former ally of Mr Putin, Mr Berezovsky became one of his fiercest critics after the pair fell out in 2000.
His fortunes as a Kremlin power-broker declined under the president and he emigrated to the United Kingdom in 2000, having survived several attempts on his life, the BBC reports.
In 2012, he lost a £3 billion damages claim against Chelsea Football Club owner Roman Abramovich, and his lawyer said he had been in a terrible emotional state after losing the case.
Mr Berezovsky claimed he had been intimidated by Mr Abramovich into selling shares in Russian oil giant Sibneft for a "fraction of their true worth".
The allegations were rejected by the London Commercial Court judge, who called Mr Berezovsky an "inherently unreliable" witness.
The oligarch's wealth is thought to have considerably diminished in recent years, leaving him struggling to pay debts in the wake of costly court cases.
Mr Berezovsky made his fortune in the 1990s selling imported Mercedes as well as Russian-made cars. He supported Boris Yeltsin's rise to power and then played a role in Mr Putin's rise in the late 1990s, before the new president moved to curb the political ambitions of Russia's oligarchs.