* 44-year-old Alexander Perepilichny found dead on Nov 10
* Had been helping Swiss investigate Russian money-laundering
* Also provided evidence linked to killing of Sergei Magnitsky
* Police say post-mortem inconclusive, more tests under way
A Russian businessman helping Swiss prosecutors uncover a powerful fraud syndicate has died in unexplained circumstances near his mansion in Britain, in a chilling twist to a Russian mafia scandal that has strained Moscow's ties with the West.
Alexander Perepilichny, 44, sought refuge in Britain three years ago and had been helping a Swiss investigation into a Russian money-laundering scheme by providing evidence against corrupt officials, his colleagues and media reports said.
He has also provided evidence against those linked to the 2009 death of anti-corruption lawyer Sergei Magnitsky, a case that caused an international outcry and prompted the United States to push for a bill cracking down on Russian corruption.
Perepilichny, a Russian citizen, collapsed and died not far from his home on an upmarket, heavily protected estate in the county of Surrey, south of London, on Nov. 10.
He is now the fourth person linked to the Magnitsky case to have died in strange circumstances.
"It is being treated as unexplained," a police spokeswoman said. "A post-mortem examination was carried out which was inconclusive. So further tests are now being carried out."
Locals at the estate - dubbed as Britain's Beverly Hills and ringed by neatly trimmed golf courses and security check points - told Reuters that Perepilichny's body, clad in running gear, was found after dark at the top of a hill.
A shaky mobile phone video clip shot by Liam Walsh, a 24-year-old local chef, showed a motionless body of what he said was Perepilichny stretched out on the side of a deserted lane lit by the light of a lone lamp-post.
"He wasn't breathing. We had to get him on the back and start doing CPR (first aid). He was probably dead for a while," Walsh told Reuters as unmarked security cars patrolled the immaculately maintained estate.
Far beyond Russia's borders, Magnitsky's death has become a symbol of corruption in Russia and the abuse of those who challenge the authorities there.
This month the U.S. House of Representatives voted overwhelmingly to "name and shame" Russian rights violators as part of a broader trade bill, brushing off warnings from Moscow that the move would damage relations.
William Browder, a former employer of Magnitsky and a prominent London-based investor, said Perepilichny had come forward in 2010 with evidence involving the Magnitsky case that subsequently helped Swiss prosecutors open their investigation.
"Alexander Perepilichny approached us in 2010 as a whistleblower with evidence about the complicity of a number of Russian government officials in the theft of $230 million which Sergei Magnitsky had uncovered," said Browder, founder of Hermitage Capital Management.
"He provided us with copies of many of the original bank documents. In January 2011, Hermitage filed an application to the Swiss authorities seeking an investigation. It was announced in March that the Swiss prosecutor's office opened an investigation and froze the assets in a number of accounts."
Browder, whose grandfather was the general secretary of the American Communist Party, was one of the biggest Western investors in Russia but was barred from Russia in late 2005 and most of his staff left the country as Hermitage found itself coming under increasing official pressure.
Magnitsky was jailed in 2008 on suspicion of tax evasion and fraud, charges that colleagues say were fabricated by police investigators he had accused of stealing $230 million from the state through fraudulent tax refunds. The Kremlin's own human rights council has said Magnitsky was probably beaten to death.
News of Perepilichny's death initially appeared on Wednesday in a report in Britain's Independent newspaper, which is backed by Alexander Lebedev, a Russian billionaire who has spoken out publicly against the Kremlin.
British media said Perepilichny appeared to be in good health before he collapsed in the evening outside St George's Hill, one of Britain's most exclusive estates, where he was renting a house for 12,500 pounds ($20,000) a month.
St George's Hill is home to many big names in the financial and celebrity circles, its long list of one-time tenants including Elton John and Ringo Starr.
Leaked secret diplomatic cables from the U.S. embassy in Moscow once described Russia as a "virtual mafia state", and London has long been the chosen destination for Russians seeking refuge from trouble at home.
But concerns have been growing in recent years that Britain might be turning into a playground for Russian mobsters as gangland violence seems to be spilling over Russian borders.
In April, a former Russian banker was shot near London's Canary Wharf financial district, sending a chill through the immigrant community. In 2006, former Russian spy Alexander Litvinenko died after drinking tea poisoned with polonium-210.
Asked about Perepilichny's case, Swiss prosecutors said it started its criminal investigation in March 2011 following a complaint made by London law firm Brown Rudnick filed on behalf of Hermitage Capital Management.
"Concerning the death of Mr Perepilichny and its consequences on the criminal proceedings, we'd like to stress that our strength resides in our ability to minimise the influence of such a regretful event on our investigation," the Swiss Office of the Attorney General said in a statement.
"A good cooperation with other judicial authorities is also essential to carry on our investigation efficiently."
Perepilichny was also a witness against Russia's notorious Klyuyev Group, a murky network of officials and underworld figures implicated in tax fraud who used European bank accounts to buy luxury property in Dubai and Montenegro, The Independent reported.
"Perepilichny was the guy who brought all the evidence they needed to open the investigation," a source told The Independent. "He brought with him records of shell companies, Credit Suisse accounts, property transactions. The whole lot."