The verdict returned by the court in southern Russia ended a yearlong trial into the September 2004 hostage-taking that survivors and relatives of those who died say has left essential questions unanswered.
The attack killed 331 people, more than half of them children, as well as 31 suspected militants and 11 elite special forces soldiers. Most of the victims died when explosions tore through the school and security forces stormed the building.
Prosecutors had demanded the death penalty for Nur-Pashi Kulayev, but Russia imposed a moratorium on capital punishment when it joined the Council of Europe a decade ago, according to the AP.
Asked whether he understood the verdict, Kulayev, a Chechen, nodded his freshly shaved head up and down. His lawyer said later Kulayev plans to appeal.
The judge said that Kulayev detonated a bomb that harmed hostages and government troops. He said 16 male hostages whom the militants executed on the first day of the assault had died in part due to Kulayev's actions, the AP reports.
Kulayev was also found guilty of shooting children and other hostages who tried to escape the school on the chaos-filled third day of the crisis. He had claimed in court that while he participated in the raid, he did not kill anyone.
Deputy Prosecutor-General Nikolai Shepel, who led the government's case, said he was satisfied with the verdict.
The group said investigators had not probed who was responsible for a chain of alleged errors: the failure to take security measures in spite of a heightened danger of terrorist attacks, the refusal to negotiate with the hostage-takers, underreporting the number of hostages involved early in the crisis, the lack of preparation for storming the school, the unpreparedness of the rescue services and the "uncontrolled use of tanks, flame-throwers, grenade-launchers and other weapons."
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