Activists plan to present evidence Tuesday that a Russian lawyer who accused officials of corruption died after a brutal beating by prison guards, saying investigators' findings that a lack of medical treatment killed the man fell short of the full truth.

Sergei Magnitsky, a 37-year-old lawyer who worked for a large U.S. investment fund, died in prison in November 2009 after the pancreatitis he developed there went untreated. He had been arrested by Interior Ministry officials after he had accused them of using false tax papers to steal $230 million from the state.

Magnitsky's case is being scrutinized by human rights activists and potential Western investors as a gauge of the Kremlin's commitment to addressing corruption and allowing an independent legal system. Several prison officials were fired but no one has been charged either for his death or in the alleged tax fraud.

On Monday, the federal Investigative Committee acknowledged for the first time that the lack of medical help was the "direct cause" of Magnitsky's death.

A panel of Russian rights activists that investigated Magnitsky's death, meanwhile, is presenting their report Tuesday to Russian President Dmitry Medvedev.

Valery Borshchev, a prominent activist who took part in the independent probe, said the Investigative Committee's acknowledgment that Magnitsky was denied medical help was a positive sign, but added that the committee kept silent about prison guards beating the lawyer.

He said the panel had found evidence that Magnitsky had died shortly after eight prison guards handcuffed him, took him to a cell and beat him with truncheons.

"We have concluded that he died of beating," Borshchev told The Associated Press. "It was a real torture to beat an ailing man with truncheons."

Borshchev said a prison doctor had claimed that Magnitsky had a nervous breakdown and that she called prison guards to calm him down until an ambulance arrived. The ambulance came quickly but prison officials kept the crew waiting at the prison gates for more than an hour while the guards were beating Magnitsky.

"Magnitsky was in a cell with eight guards who were beating him for more than one hour, and there were no doctors around," Borshchev told the AP on Monday. "Bruises were found on his body and there are pictures of his broken knuckles, testifying that he was beaten."

The ambulance crew said Magnitsky had died 15-30 minutes before their arrival, while the official reports claimed he died an hour later and lied that he died at a prison hospital, Borshchev said.

Officials at the Investigative Committee couldn't be reached for comment on Borshchev's claim.

The Investigative Committee said Magnitsky also had cardiomyopathy, a form of heart disease, and it was the combination of this and his other illness in the absence of medical aid that caused his death.

The agency said it has identified those responsible for Magnitsky's death but will name them later.

Magnitsky had been charged with tax evasion linked to his defense of Hermitage Capital Management, a multibillion-dollar fund headed by U.S. businessman William Browder, who has since been deported from Russia as a national security threat.

Hermitage accused Interior Ministry officers of seizing ownership documents of three of its subsidiaries in 2007, then using those documents to register their own people as owners and directors. The Interior Ministry officials then filed a tax claim, saying they made a much smaller profit than originally described and asked for a tax return, according to Hermitage.

The total return for the three subsidiaries was 5.4 billion rubles ($230 million at the time).

Browder says a Moscow tax official who approved the fraudulent tax return bought luxury real estate in Moscow, Dubai and Montenegro and wired $39 million through her husband's bank accounts. All that was done on an average annual Russian household income of about $38,000, according to documents released by Browder.

Lawmakers in the Netherlands on Monday voted in favor of a resolution demanding the government slap a travel ban on dozens of Russian officials who Browder and his supporters consider complicit in Magnitsky's death. The U.S. Senate is working on similar legislation.

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Vladimir Isachenkov in Moscow contributed to this report.