The report by employer Hermitage Capital concludes Sergei Magnitsky, 37, was left to die in his cell in 2009 after suffering brain trauma from a beating, The Moscow Times reported Tuesday.
The 75-page document includes morgue photos taken of Magnitsky showing bruises on his wrists and legs.
It reproduces what it said is a photocopied order from the head of the facility where Magnitsky was being held directing guards to beat him with a rubber baton.
Magnitsky, who accused officials of stealing $230 million in government money, was awaiting trial on tax charges at the time of his death.
Two prison doctors have been charged with his death.
The Hermitage report has been given to Russia's Investigative Committee.
The Kremlin's human rights council has backed the report but government officials have not issued any comments.