WEYBRIDGE, England -- A Russian businessman helping Swiss prosecutors uncover a 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 led to an international outcry and prompted the United States to push for a Russian crackdown on corruption.

Perepilichny, a Russian citizen, collapsed and died not far from his home Nov. 10 on an upmarket, heavily protected estate in Surrey, south of London.

He is the fourth person linked to the Magnitsky case to die in strange circumstances.

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"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, ringed by neatly trimmed golf courses and security check points, said that Perepilichny's body, clad in running gear, was found after dark at the top of a hill.

A shaky cellphone video clip shot by Liam Walsh, 24, a local chef, showed a body he said was Perepilichny stretched out on the side of a deserted lane.

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.