A tax lawyer's death in a Russian prison in 2009 unleashed a diplomatic storm between Moscow and Washington, and his accusations of fraud carried out by Russian officials have now reached French shores.

Sergei Magnitsky became a symbol of the fight against corruption in Russia, with his supporters saying that he was unjustly imprisoned and left to die on purpose.

Magnitsky had accused interior ministry officials of organising a $235 million tax scam against the investment company Hermitage Capital, but was then charged with the very crimes he said he had uncovered.

He was placed under pre-trial detention in 2008 and died of untreated illnesses less than a year later.

His American former employer Bill Browder, a major financial player in Russia before he fled in 2005, has been testifying to French judges investigating allegations that some of the funds were laundered through France.

French police last year opened an investigation, which has since been passed on to three judges.

Of particular interest for investigators has been a mysterious company in Saint Tropez on the French Riviera apparently specialising in the export to Russia of high-end French and Italian paints.

The owner, a 58-year-old woman with dual French and Russian citizenship, was charged with fraud and embezzlement last month, a judicial source told AFP.

A probe into the company's accounts found transfers of more than 16 millions euros from the British Virgin Islands and Belize between 2008 and 2013.

The woman in question, who police declined to name for security reasons, enjoys "a high-flying lifestyle", a source close to the investigation said.

The source said that the woman helped organise a system for embezzling Russian public funds through tax scams and shell companies in tax havens.

- Browder's crusade -

The investigation also points to the leading role played by an as yet unidentified woman named "Tatyana", who allegedly took over the position from a brother assassinated by business associates.

Browder said that the embezzled money trail from Hermitage leads from Russia to several Moldovan banks through the British Virgin Islands and Belize and then across Europe including to France.

A Moscow investment banker, Alexander Perepilichny, proved particularly helpful to Browder in untangling the web, providing him with bank records in July 2010.

In 2012, after returning from a few days' stay in the French capital, Perepilichny died at the age of 44 after going running near his villa in Surrey, southeast England.

British police concluded that the death was not a suspicious but media reports cited a lawyer for his life insurance company saying that test results on his body had found a chemical linked to a rare poison.

Known as "heartbreak grass", the Gelsemium elegans plant grows only in Asia and high doses can cause paralysis of the spinal cord, a loss of muscular power and eventually death from asphyxia.

Browder has told French judges that he believes Perepilichny was poisoned in Paris, sources said.

Browder said he and Hermitage lost political support in Russia following the brutal demise of former oil tycoon Mikhail Khodorkovsky in 2004.

Chicago-born Browder, a grandson of a former leader of the American Communist Party, followed the raid against his company from London in 2007.

Magnitsky went public with the alleged tax scam involving Hermitage in 2008. Just months later he was dead, with an official Russian investigation finding evidence of beatings and healthcare being withheld.

Browder in 2012 persuaded US lawmakers to sanction officials allegedly linked to the case by banning them from the United States and freezing their assets.

He has since continued his crusade, trying to persuade other countries to join the sanctions.