An independent probe into the 2009 prison death of Russian lawyer Sergei Magnitsky says prison guards beat him to death with rubber batons and the government covered it up.

The report was drawn up by William Browder, founder of the British-based investment fund, Hermitage Capital Management. Magnitsky worked for Hermitage before he was arrested on what supporters say were phony tax evasion charges. Magnitsky had accused police officials of using Hermitage as part of a scheme to steal hundreds of millions of dollars.

Browder’s report says guards beat Magnitsky with batons and he died about an hour later. It also accused the government of failing to punish those who killed him as well as those who embezzled the money

The report contradicts an investigation by Russian President Dmitry Medvedev’s human rights council, which said Magnitsky died from an inflamed pancreas which it said he had before entering prison a year earlier. Russia has charged two prison doctors with negligence for not treating the illness.

The Russian government has not yet commented on the Bowder report.

The United States has denied U.S. visas to a number of Russian officials involved in the Magnisky case. Russia imposed similar measures against certain U.S. officials last month.