Surrey police are carrying out a second postmortem on a Russian whistleblower found dead outside his Surrey home, three years after he fled to the UK to escape a Russian crime syndicate.

Detectives said they were awaiting the results of toxicology tests on 44-year-old Alexander Perepilichnyy. An earlier postmortem was "inconclusive", they said. The Russian businessman, who had appeared in good health, was found collapsed earlier this month outside his luxury mansion in Weybridge. He was discovered at 5.15pm and pronounced dead 25 minutes later. Witnesses said he was wearing shorts and running shoes. According to his wife, he had been for a run.

Police are so far describing the death as "unexplained". But Perepilichnyy was a man with enemies. He had accused Russian government officials of complicity in one of the biggest tax scandals in Russian history. The case involved the interior ministry and the alleged theft of $230m (£144m) in taxes paid by the Hermitage investment fund to the Russian government. A Russian lawyer who uncovered the scam, Sergei Magnitsky, was jailed by the authorities and died in prison three years ago.

According to Hermitage, Perepilichnyy was a key witness against one of Russia's most notorious criminal gangs, the "Klyuev group". The group's members include Russian officials in law enforcement, the FSB spy agency, the tax agency and senior judges. Its members are allegedly responsible for a series of multimillion dollar tax frauds, dating back over a decade and including the Magnitsky case. They are said to have siphoned off $800m. Perepilichnyy is the fourth person linked to the group to have died a sudden death.

Bill Browder, Hermitage's chief executive, who has campaigned to expose Russian corruption, confirmed that Perepilichnyy was a major whistleblower. He said the Russian businessman approached Hermitage in 2010 with evidence about the complicity of Russian government officials in the Magnitsky theft. Browder said: "He provided us with copies of many of the original bank documents."

In January 2011, Hermitage filed an application to authorities in Switzerland based on Perepilichnyy's evidence, which included details of accounts held at Credit Suisse. In March, the Swiss prosecutor's office opened an investigation and froze the assets in a number of accounts linked to the "Klyuev group", members of which have bought properties in Dubai and Montenegro. They allegedly laundered criminal proceeds through the international banking system using banks and companies in Cyprus, Switzerland, Moldova and the UK.

Writing in his blog on Wednesday, the Russian anti-corruption campaigner Alexey Navalny said it was Perepilichnyy who had exposed the corrupt Russian tax officials involved in the Magnitsky case. He said he was waiting to find out the causes of Perepilichnyy's death. He added: "There were no signs of a violent death. But there weren't any in connection with [Alexander] Litvinenko either."

The Kremlin has been determined to prevent the UK and other EU countries from introducing the "Magnitsky Act", which seeks to name and shame the 60 Russian officials implicated in Magnitsky's death. The US House of Representatives last week voted to ban the officials from entering the US and to freeze their US assets. The Obama administration opposes the proposed Magnitsky act, fearing it will derail relations with Moscow. Britain has also refused to publicly adopt it, despite cross-party calls in the House of Commons for the list to be made law.

A detective chief superintendent from Surrey's major crimes squad has now been appointed to investigate the death, it emerged on Wednesday. Surrey police said: "We continue to treat the death of a man in his 40s who collapsed in Weybridge on Saturday, 10 November as unexplained. The death remains under investigation whilst officers await the results of the postmortem and toxicology tests. Surrey Police continues to work closely with the coroner's office whilst the investigation remains ongoing."