From Andrew Osborn in Moscow

The only two non-Kremlin-aligned MPs to take part in Russia's parliamentary inquiry into the 2004 Beslan school siege have broken their silence to denounce the investigation as a cover-up that did little more than go through the motions.

According to nationalist Yuri Savelyev, and Communist Yuri Ivanov, Russia's worst post-Soviet act of terrorism was deliberately investigated poorly, in order not to undermine the Kremlin's official version of events, an account that both men believe was fabricated.

In the aftermath of the tragedy, President Vladimir Putin promised the relatives of those who were killed that a painstaking official investigation would establish what really happened on September 1-3, 2004.

Some 333 people died in the siege, more than half of them children, when pro-Chechen terrorists took more than 1000 people hostage in a small school in southern Russia. Many of those who survived felt the Kremlin's handling of the crisis was negligent and demanded a proper explanation.

Yet to survivors' disbelief, the parliamentary investigation into the tragedy was quietly wound up at the end of last year after almost two years of work without a murmur.

According to the two whistle-blowing MPs, that inquiry was little more than a stage-managed PR exercise.

"It was nonsensical, shameless, and betrayed a complete lack of conscience," said Ivanov, who added he and Savelyev had refused to sign off on the report because of their misgivings.

Ivanov claimed the investigation was warned not to call Putin as a witness and that the report was rushed through parliament "within 20 minutes".

While it did criticise the action of local authorities on the scene, it generally shied away from questioning the way in which the siege was handled and focused instead on the guilt of the Chechen hostage-takers.

Both MPs said their work on the inquiry had prompted them to seriously question the official version of events.

They alleged that the siege's bloody climax was not triggered by Chechen terrorists detonating explosives inside the school as the Kremlin claims, but by rocket-propelled grenades being fired at the school from an area under the control of Russian special forces.

Ivanov further contended that the grenades were fired on the direct orders of President Putin as a prelude to the storming of the school.

Savelyev, a member of the nationalist Rodina (Motherland) party, said his work on the panel had forced him to reassess what happened in Beslan.

"When I started work in October 2004 I sincerely believed that the terrorists had raped and killed hostages, blown up the school, and then shot fleeing women and children in the back."

But he said now he had reached a very different conclusion. He said the panel did not turn up any substantive evidence to suggest that the terrorists had detonated explosives inside the school or that they had shot fleeing hostages in the back. Allegations of rape have also yet to be substantiated.

The two MPs also claimed that the authorities destroyed crucial evidence in the first few hours after the siege was broken. They accused the government of deliberately allowing a fire in the school to rage for almost two and a half hours before allowing the fire brigade to enter, in a calculated cover-up.

By the time the fire brigade did get access, the roof of the school gym had been totally destroyed and with it crucial evidence that the MPs believe would have shown that rocket-propelled grenades had been fired into the gym through the roof.

The Kremlin has dismissed the pair's claims, insisting the parliamentary inquiry was "serious and unbiased." However, local people in Beslan say they are more inclined to believe the two MPs than the Kremlin version.

"Evidently there are people keen to ensure that the truth about Beslan is never known," said Ella Kesaeva of the Voice of Beslan pressure group, who lost her two nephews in the siege.

"But we will do everything we can to establish why our children and loved ones died and who was responsible for their deaths."