Chemical and radiation experts are assisting the police after exiled Russian businessman Boris Berezovsky was found dead in a bath at his home in the UK.
It is believed the former billionaire, who had fallen out with the Kremlin, took his own life, but this has not been confirmed.
Thames Valley Police said it had launched an investigation into the death of the 67-year-old at his property in Ascot, Berkshire.
His death was "currently unexplained" and a "full inquiry under way", police said.
A police statement said: "Specially trained officers are currently at the scene, including Chemical, Biological, Radiological and Nuclear (CBRN) trained officers, who are carrying out a number of searches as a precaution.
"The body of the man is still in the property at this time."
Mr Berezovsky's lawyer Alexander Dobrovinsky told Russian state television he had been informed by contacts in London that Mr Berezovsky had killed himself.
He said: "Berezovsky has been in a terrible state as of late. He was in debt. He felt destroyed. He was forced to sell his paintings and other things."
Mr Berezovsky made his fortune in Russia in the 1990s when he bought up state assets which were being sold off cheaply.
Mr Berezovsky had lived in Great Britain from 2000, having fled from Russia after falling out with the leadership.
He was also a friend of murdered dissident Alexander Litvinenko, who died in London after consuming radioactive polonium in 2006.
In 1997 Forbes magazine estimated Mr Berezovsky's wealth at \$3bn (£2bn), but in recent years his wealth had been considerably reduced.
It is thought he had done badly in the financial crisis. In 2009 his wealth was estimated at £450m, but he is thought to have spent £100m on the £3.7bn lawsuit against Chelsea owner Roman Abramovich last year, which he lost.
James Nixey, head of Chatham House's Russia programme, said: "He is the most virulently anti-Kremlin, anti-Putin of the oligarchs.
"He was certainly willing to spend his money, what little he had left, in an attempt to use it to end the current regime in Russia.
"He had bodyguards, there were attempts on his life that even the security service in the UK had warned him about.
"It's certainly not the first case of Russians and people from the former Soviet Union, more broadly, who have been involved in difficult, embarrassing disputes with the Kremlin, to have died in relatively mysterious circumstances."
However, speaking to Sky News, a friend of Mr Berezovsky said she did not think his death was suspicious.
Sasha Nerozina said: "There is nothing to be suspicious about, as far as I understand.
"It is shocking, terrible news. It is not something you expect. He was full of life and love ... we expected him to outlive us all."
She said Mr Berezovsky had been left "demoralised" by losing the high-profile legal battle with Mr Abramovich last year.
Vladimir Putin's spokesman Dmitry Peskov told Russian state TV that Mr Berezovsky had recently written to the President to ask for a pardon and to say that he wanted to return to Russia.
He said: "He asked Putin for forgiveness for his mistakes and asked him to obtain the opportunity to return to the motherland."
Lord Truscott, who has written a biography of Mr Putin, said: "Was it suicide or was it murder? He had a lot of contact with people in Russia. There could be a whole host of people who could want to see him dead.
"Last year he lost a case against Mr Abramovich and was getting very short of money. He could have been in a depressed state. Perhaps it was a final desperation."
A cordon remains around the property while police continue their investigation.
Superintendent Stuart Greenfield said: "We are aware the cordon is causing disruption to local residents and we apologise for any inconvenience.
"But it is important we take all necessary measures to ensure a full and thorough investigation can be carried out.
"I would like to reassure residents that we are confident there is no risk to the wider community.
"The property is part of a large estate so a number of roads are closed off at the moment and will remain so for the time being."