A Moscow court on Friday acquitted the only official charged in connection with the 2009 death of jailed Russian lawyer Sergey Magnitsky, the news agency Interfax reported.
The case has severely strained relations with the United States, culminating this week in the passing of a Russian bill banning US citizens from adopting Russian orphans.
The court ruled that the deputy head of the prison, Dmitry Kratov, was not guilty of neglect and there was no connection between Kratov's actions and the death of Magnitsky.
The 37-year-old lawyer, who was imprisoned on tax evasion charges in November 2008, had been suffering from untreated pancreatitis when he died a year later. Prison officials said he died of heart failure.
An inquiry ordered by the Kremlin's human rights council alleged that he had been badly beaten hours before his death, according to Ria Novosti.
He was arrested after exposing alleged tax fraud worth hundreds of millions of dollars involving companies linked to the Kremlin.
The US in December passed the Magnitsky Act, which imposes sanctions against Russian officials suspected of involvement in human rights violations.
Russia called the bill "an unfriendly act towards the Russian Federation" and retaliated by passing the adoption bill, which President Vladimir Putin has said he will sign into law.