Sergei Magnitsky, 37, died in November 2009 after being beaten and denied treatment for pancreatitis, concluded the Council for Civil Society Institutions and Human Rights, appointed by President Dmitry Medvedev.
But the Interior Ministry will not press charges against the investigators responsible for his imprisonment, The Washington Post reported Tuesday.
The prosecutor's office is considering reopening the tax fraud case against Magnitsky that was closed when he died, which might clear his name.
But Dmitri Kharitonov, the Magnitsky family's lawyer, told the Interfax news agency: "I do not trust our investigative bodies. Try as I might, I cannot imagine them admitting all of a sudden that they kept an innocent behind the bars."
Magnitsky was arrested after he accused police and tax officials of a $230 million tax fraud. Instead he was charged with stealing the money and mistreated in prison to make him implicate executives of Hermitage Capital, independent investigators say.