A Russian investigator has said grenades fired by surrounding Russian forces could have triggered the Beslan school bloodbath in September 2004.
The gunmen crammed the hostages into the school gym
Yuri Savelyev's conclusions contradict the official view that bombs planted by the hostage-takers in the school gym went off just before the gun battle.
Mr Savelyev is a member of the Russian parliamentary commission investigating the siege, in which 331 people died.
Many of the victims were children, taken hostage by pro-Chechen militants.
In an interview with Moscow Echo radio on Monday, Mr Savelyev, a weapons and explosives expert, said that during the investigation, he "discovered that the consequences of those blasts could not at all be explained by the explosions of the home-made devices installed by the rebels".
"Most of the hostages were talking about explosions in a totally different part of the gym from that to which the official investigation referred.
"As a result, I came to the conclusion that these home-made explosive devices installed by the rebels did not explode at all. Those were explosive devices delivered from outside," he said, adding that it could have been "shots fired from grenade-launchers".
The mourning continues for the mothers of the dead
He said the explosions killed many of the hostages and dozens more died in the resulting fire.
Many relatives blame their children's deaths on the botched rescue operation, in which fire engulfed the school, in the Russian Caucasus republic of North Ossetia.
A North Ossetian parliamentary commission said the school had been seized because of "failings in the law enforcement bodies".
Row rages on
The head of the commission, Stanislav Kesayev, said he had confidence in Mr Savelyev's conclusions.
"He had more resources than our commission. He relied on his own knowledge as a weapons specialist and mathematician," Mr Kesayev told the radio.
Mr Savelyev's conclusions were published on the website pravdabeslana.ru.
One of his colleagues on the Russian parliamentary commission, Arkady Baskayev, rejected his conclusions.
He said there was "nothing convincing in the trajectory of these special shells" which Mr Savelyev blamed for the explosions.
"This is Mr Savelyev's private opinion, which is not confirmed in any way," he said.
The earlier North Ossetian investigation concluded that grenade launchers, flamethrowers and tank fire had been used during the storming of the school by Russian security forces.
Russia's prosecutor general admitted such equipment had been used, but only after all the children had left the school.
For weeks after the siege Russian officials had denied the use of flamethrowers.