A Russian businessman linked as a witness to a high-profile corruption scandal has been found dead near his home in Weybridge.
Alexander Perepilichnyy had collapsed on a road early on the evening of 10 November, Surrey Police said.
Police are treating the 44-year-old's death as unexplained, pending toxicology tests after an inconclusive post-mortem examination.
Reports have connected Perepilichnyy to the Sergei Magnitsky affair.
Magnitsky, a lawyer for London-based Hermitage Capital Management, died on remand in a Moscow prison three years ago after allegedly uncovering a web of corruption involving Russian tax officials.
According to an article in the Independent newspaper on Wednesday, Perepilichnyy had been giving evidence to Swiss investigators about Russian fraud involving Swiss-based bank accounts.
He had sought sanctuary in the UK three years ago after "falling out with a powerful crime syndicate", the paper said.
A Russian media report described Perepilichnyy as a former business partner of one of the people accused by Magnitsky of fraud.
'All the evidence'
News of Perepilichnyy's death more than two weeks ago only emerged on Wednesday.
Police were called to Granville Road in Weybridge shortly after 17:15 GMT, following a report a man had collapsed there.
An ambulance had attended, but Perepilichnyy had been pronounced dead just before 17:40 GMT, Surrey Police said. The force would not confirm suggestions he had been out running.
"We were made aware of Mr Perepilichnyy's link as a witness in an ongoing trial during the course of the investigation," the police said.
According to an article in the Daily Telegraph newspaper, Perepilichnyy had lived on a "luxury private estate shared with seven multi-million pound properties".
The Swiss Attorney General's office told the Independent Perepilichnyy had given evidence to federal prosecutor Maria Antonella-Bino.
The paper quoted a "source with knowledge" of the Swiss fraud inquiry as saying Perepilichnyy had "brought all the evidence they needed to open the investigation".
Evidence he had reportedly provided included records of shell or "front" companies - which serve as vehicles for business transactions without themselves having any significant assets or operations - bank accounts and property transactions.
Magnitsky had uncovered the alleged theft by Russian tax officials of more than $200m (£125m).
Himself detained on suspicion of tax evasion, he died in custody aged 37.
His cause was taken up by human rights groups as one of the most glaring examples of corruption and prison abuse in modern Russia.
He had suffered from pancreatitis and gallstones, and had been found with broken fingers and bruising to his body, the Kremlin's Human Rights Council said in July 2011.
There were, it said, grounds to suspect that he had died as the result of a beating.