Aleksandr Torshin was reading his commission's preliminary conclusions to the upper house of parliament, 16 months after the tragedy.
Torshin said that Russian Interior Minister Rashid Nurgaliev and his deputy had sent telegrams less than two weeks before the hostage-taking instructing local police to hike security ahead of the first day of school on 1 September 2004.
But he said only one police officer was posted outside Beslan's school No. 1 when militants attacked, and she was taken hostage along with over 1,100 parents, children, and teachers.
Torshin said local authorities committed a number of mistakes during the three-day hostage crisis.
"The approximate number of hostages became known already on the afternoon of 1 September [the day the school was seized], however [local] officials continued to report to the government for one more day that 354 hostages were being held," Torshin said. "Coordination among law-enforcement personnel was weak. The cordon failed to hold back Beslan residents when they forced their way toward the building trying to save the children and opened fire at the school. The list of errors and failures goes on."
Torshin also said the mechanisms to prevent terror attacks were inadequate and that many law-enforcement officials had no crisis training.
The school siege ended on 3 September in a chaotic battle between Russian forces and the hostage takers, who were demanding Russia withdraws its troops from Chechnya. More than 330 hostages, most of them children, were killed in the bloody standoff.
According to the official version of events, Russian forces launched a spontaneous attack to free hostages after an explosion inside the school. Officials say most of the victims died in the firefight that followed.
But relatives of the victims accuse the authorities of botching up the rescue operation and of trying to cover up mistakes.
Anetta Gadiyeva, who was among the hostages and lost her daughter in the siege, watched excerpts of Torshin's speech on television from Beslan. She told RFE/RL she was happy with what she'd seen so far: "The fact that Torshin assesses the headquarters and the performance of all those who were meant to free the hostages, saying the work was badly coordinated, is honorable. Of course we are interested in hearing the rest of the report, looking at it, and then we will be able to evaluate it. But so far, the fragments we have seen have not disappointed us."
The commission's findings contrast sharply with a report released on 27 December by the Prosecutor-General's Office.
Deputy Prosecutor-General Nikolai Shepel said on 27 December the investigation had found no mistakes in the authorities' response to the hostage-taking.
His remarks sparked angry reactions from relatives of the victims, who say official blundering made the bloodshed worse.
Relatives have repeatedly accused special forces of using flame-throwers, grenade launchers, and tanks against militants while there were still hostages alive in the school -- a claim authorities have consistently rejected.
In his preliminary report, however, Torshin supported the official version on the use of heavy weaponry.
But he said many other contentious points remained hazy, and added he was still waiting for the Prosecutor-General's Office to provide him with documents that could help shed light on the events.
Russian President Vladimir Putin ordered the Prosecutor-General's Office to launch an inquiry into the tragedy, but initially resisted the idea of a probe by the upper house of parliament.
A third investigation led by North Ossetia's parliament said the rescue efforts were severely flawed and called for guilty officials to be punished.