Russian President Dmitry Medvedev has ordered an investigation into the death in prison of a lawyer who was awaiting trial on charges of tax evasion.
Sergei Magnitsky, 37, who was being held on suspicion of conspiracy, died last week from what investigators said was acute heart failure.
It has since emerged he had repeatedly complained that prison authorities had refused to allow him medical care.
He acted for a company that was once Russia's top foreign investor.
Hermitage, the brainchild of US businessman William Browder, was one of the largest investment funds in Russia from the mid-1990s.
Mr Browder said the Russian authorities withheld medical treatment from Mr Magnitsky because he refused to sign a confession.
Mr Medvedev had ordered Russia's chief prosecutor and justice minister to investigate the case, said a Kremlin statement.
Deaths in custody, sometimes involving ill-treatment, are not uncommon in Russia.
But Mr Magnitsky's role in defending Hermitage means that the case has drawn wider attention and immediate comment.
Mr Magnitsky had been arrested on suspicion of conspiracy after agreeing to defend Hermitage against alleged tax evasion.
In 2005, Mr Browder was banned from Russia as a threat to national security, after Hermitage publicised accusations of large-scale corruption in state corporations.
Mr Browder says he was punished for being a threat to corrupt politicians and bureaucrats.