Russian investigators say the mass hostage-taking in Beslan, which led to hundreds of deaths, was carefully planned by Chechen rebel leaders.
Beslan's School Number One was gutted by fire during fierce fighting
The Beslan school siege in September 2004 killed at least 331 people, many of them children.
The Russian parliamentary commission said the pro-Chechen gunmen who seized the school included a bodyguard of the late Chechen warlord Shamil Basayev.
The local North Ossetian police were also blamed for security shortcomings.
The commission chairman, Alexander Torshin, reported that not a single armed policeman was on duty outside the school on the day the gunmen assaulted it.
The commission concluded that 32 militants took part in the raid and that they set off the first explosion inside the school.
According to the Torshin commission, the attack on the school was planned well in advance by top Chechen rebels Aslan Maskhadov and Shamil Basayev - both of whom were later killed - and another militant called Abu Dzeyt.
The gunmen filmed the hostages' nightmare in the school gym
The commission rejected claims by victims' relatives that Russian security forces deliberately stormed the school to flush out the gunmen.
The commission's conclusions contradict an earlier account of the siege given by Russian investigator Yuri Savelyev.
He said grenades fired by surrounding Russian forces probably triggered the bloodbath, arguing that the explosive devices planted by the gunmen "did not explode at all".
Many relatives blamed their children's deaths on the botched rescue operation, in which fire engulfed the school, in the Russian Caucasus republic of North Ossetia.
A campaigner representing victims' relatives, Ella Kesayeva, voiced anger at the Torshin commission's findings.
Speaking on Ekho Moskvy radio, she said the report gave the Russian authorities the green light to "use banned weapons with impunity".
"The next time hostages are taken the security services can do whatever they like, they can do what they did in Beslan," she said.