The UN Special Rapporteur on Torture published a report Tuesday condemning the Russian government over the death of lawyer Sergei Magnitsky.
The report by Juan E. Mendez said the state should take responsibility for Magnitsky's death in prison, since he was healthy before his incarceration.
"When an individual dies as a consequence of injuries sustained while in State custody, there is a presumption of State responsibility, particularly when the person was in good health at the time of his arrest," the report said.
Magnitsky was arrested in 2008 on charges of tax evasion, after he began investigating Interior Ministry and tax service officials in an alleged fraud case. He spent 11 months in prison without trial and reportedly developed a number of serious illnesses, for which he was denied treatment. He died eight days before he was legally required to be released if he were not put on trial.
Western governments have harshly criticized Russia over the case, with some countries considering so-called "Magnitsky acts" barring entry to officials believed to have been involved in the lawyer's death. Magnitsky's former employer, Hermitage Capital, has conducted a lobby campaign to pressure the Russian government to investigate the case.
Russian authorities have declined to investigate the torture allegations, claiming that the 37-year-old died of natural causes. Mendez's report said such arguments were difficult to believe.
"Attempts to present the cause of death as arising from natural causes are particularly unpersuasive given the medical records available and the fact that urgent medical attention was denied to him as his condition deteriorated," Mendez wrote.
Mendez, who previously worked as a professor of human rights law at Oxford University, also criticized the investigation into the case, which Hermitage Capital said has been extended 11 times in over two years, with no one prosecuted. Mendez added that Magnitsky's family has been put under duress.
"Although officials presumably involved in Magnitsky's arrest and treatment in detention have been identified, their conduct has not been properly investigated," the UN report wrote. "Instead, Magnitky's relatives are being asked to contribute evidence or be portrayed as not being interested in the investigation."
The report said Russia is obliged to comply with the UN Anti-Torture Convention, which Mendez said it has failed to do.
Last week, Magnitsky supporters were refused permission to conduct a rally demanding a fair investigation into the case. The protest was not allowed on the grounds that it "may influence the judicial process."