RIGA (Reuters) - Latvia is to investigate whether the country's banks played any part in an alleged multi-million dollar Russian tax fraud made public by Sergei Magnitsky, a lawyer who died in detention in Russia in 2009.

The state prosecutor's office said on Wednesday it had reversed a decision by police to reject a complaint from British law firm Brown Rudnick alleging Latvian banks were used to launder $19 million.

Prosecutors have checked material linked to the case and sent the complaint back to the police for further investigation as it was rejected without considering the facts presented.

In July, Latvian authorities received a letter from Brown Rudnick, acting on behalf of London-based Hermitage, once the largest foreign investor in Russia and for whom Magnitsky worked, alleging six Latvian banks had been involved in laundering $19 million.

The case stems from whistleblowing by Magnitsky, who died just under a year after being held on tax evasion and fraud charges which, former colleagues said, were fabricated by police investigators he had accused of stealing $230 million from the Russian state through fraudulent tax refunds.

While Magnitsky's death was officially attributed to an undetected illness, the Kremlin's own human rights council has said he was probably beaten to death.

U.S. lawmakers have drafted legislation named after Magnitsky that would impose sanctions on Russian officials involved in human rights violations.

Russia strongly opposes the bill, saying it would have disastrous consequences for the U.S.-Russian relationship.

(Reporting by Aleks Tapinsh; Editing by Dan Lalor)