Police investigating the death of a whistleblower found dead in Surrey were unwilling to believe that the Russian mafia may have been involved and "brushed off" attempts to explain that he had been at risk from an international organised crime syndicate, it has been claimed.

Since the body of Alexander Perepilichnyy was found outside his home three weeks ago, it has emerged that the 44-year-old had been the key co-operating witness in a Swiss investigation of a multimillion-pound tax fraud in Russia, linked to individuals suspected of involvement in the death of lawyer Sergei Magnitsky.

The claim that police were slow to realise the possible implications of the death of Perepilichnyy was made by businessman Bill Browder, who had hired Magnitsky to investigate a massive tax fraud perpetrated on his company, Hermitage Capital Management. He was aware that Perepilichnyy was assisting money-laundering investigations into the same corrupt Russian officials.

Upon hearing of Perepilichnyy's death, which remains unexplained, Browder contacted Surrey police, explaining why it was crucial that they conducted a sophisticated autopsy. Six years ago, former KGB officer Alexander Litvinenko was fatally poisoned with the radioactive element polonium-210 in London, after meeting two Russians at a central London hotel.

Browder said: "We informed the chief constable why this person [Perepilichnyy] was significant, and why they should do a very detailed autopsy using the best experts they could find."

London law firms Brown Rudnick and Peters & Peters wrote repeatedly to the Surrey force on behalf of Browder, explaining they had considerable information on "Perepilichnyy's role as an informant against Russian organised crime, fraud and corruption".

Browder added: "They contacted Surrey police multiple times to make sure they were treating this seriously, but were brushed off entirely. All our approaches were apparently ignored." He said it was only when media reports emerged last week of Perepilichnyy's death that Surrey police appeared to change tack. Since then the force has ordered a second postmortem, after the first proved inconclusive, and toxicology tests.

Surrey police said they had received information regarding Perepilichnyy a week after he died and that "a review is currently under way into the response".

It added: "The inquiry is focused on establishing the cause of death and officers are currently waiting on results from toxicology tests which are ongoing."

Perepilichnyy is understood to have provided documents that prompted a continuing international investigation into the role of Moscow tax officials allegedly at the centre of a massive fraud whose proceeds were laundered through Swiss bank accounts. His evidence is understood to have pointed the finger at officials linked to the Klyuev organised crime group, a syndicate allegedly connected to the fraud Magnitsky exposed.

Hermitage Capital Management was one of the largest foreign investors in Russia until it became the victim of a massive tax fraud in 2007. Magnitsky concluded that Russian interior ministry officials, police and organised criminals had colluded to take control of Hermitage's Russian interests and fraudulently reclaim millions in taxes already paid.

Shortly after testifying against several police officers, Magnitsky was arrested and imprisoned at the Butyrka prison in Moscow. The 37-year-old was detained for 358 days without trial, tortured, denied medical treatment for pancreatitis and beaten with rubber batons shortly before he died on 16 November 2009.

Perepilichnyy is believed to have identified officials from Moscow who became wealthy in the immediate aftermath of the fraud and used Swiss bank accounts to buy foreign properties. At least four people linked to the Magnitsky case have died in mysterious circumstances.

Fresh details passed to the Observer reveal how, after the Magnitsky fraud came to light, officials accused of the fraud blamed a man called Octai Gasanov, 58, who it later emerged had mysteriously died weeks before the scam was perpetrated. The group then blamed a 48-year-old known as Valery Kurochkin. Train records show Kurochkin travelling to the Ukraine in March 2008 with other members of the syndicate, shortly after the tax fraud. The return journey shows all the group members but Kurochkin. He was found dead near to Boryspil airport, near Kiev, in April 2008. His death certificate says he died from cirrhosis.

There is no evidence, however, that the Klyuev syndicate was involved in the deaths, or that of Perepilichnyy.

Browder – whose team has received 12 death threats while investigating Magnitsky's death – said if it were to become apparent that Perepilichnyy had been murdered, it would underline the risk faced by those investigating organised crime in Russia, but would also suggest that the British police were insufficiently aware of the issue.

He added: "The authorities here don't seem to have the capacity to deal with this sort of situation. We make our own security arrangements. The authorities don't seem to have any appreciation of how dangerous these people are."

An interior ministry official told Russian news agency Interfax last week that Perepilichnyy had nothing to do with the Magnitsky tax case, while businessman Dmitry Klyuev denied the existence of a criminal syndicate in a letter to Vedomosti, a Russian daily newspaper.