It will be one year ago on Thursday that Chechen (search) terrorists stormed an elementary school in Beslan, Russia (search), on the first day of the school year — it's called Knowledge Day there — holding more than 1,000 kids and adults hostage for three days.

When the siege ended on Sept. 4, 171 children and more than 200 adults were dead.

The horror of the ordeal will be remembered Thursday on HBO's (search) incredibly powerful, moving documentary, "Children of Beslan," one of the most heartbreaking you'll ever see.

The film begins with home-video footage taken that very day by proud, excited parents who brought their children — dressed in traditional Russian costumes — to school.

We see gorgeous, happy kids, thrilled parents, grandparents, teachers and others on the first day of school. Every kid has colorful balloons as they mug for the cameras. When the guns started to go off, the kids thought that they were the balloons popping and kept up their playful fun.

Within seconds, however, the school was under siege and the story that is told from here on is the children's story. It is told by the survivors with some incredible video footage that was somehow taken during the siege and with footage of the kids now in their homes, in their new school and in the remnants of their former school.

They are heartbroken little kids, yes, but they are not broken kids. Actually, for the most part they aren't even kids anymore — too much happened to them.

One beautiful little boy takes the filmmakers through the bombed-out school, showing step-by-step where the terrorists held them, and then he says, "It was here that they killed my dad and threw him out the window."

The documentary recounts how the terrorists — wearing masks, carrying machine guns and strapped with bombs — took the school and then hung bombs from light fixtures in the assembly where they held the thousand hostages in the stifling heat.

On the second day, they began denying them water. One child recalls that she said, "Mister! Mister! Let us have a drink," and that he replied, "I'm not a mister — I'm a terrorist!"

Another child tells of a female terrorist who wanted to let the children have a drink, but a second terrorist blew her up in front of them, with her brains splattering on the kids.

In another incident, after the military had begun to move in and children were escaping through windows, one kid recalled that she saw an outdoor faucet with desperate children lined up to drink. She began to head over, and then "they aimed a grenade launcher at the children," and the kids on line blew up.

The siege ended three days later, but the surviving children are, for the most part, in deep depressions or suffering from extreme anxiety. One boy checks his mother's grocery bags for bombs, another lives to kill the people who killed his family and friends.

A father tells his son that God is good, but the kids seem to think otherwise.

"He kept the most best children," a girl says, "the most beautiful children died," while another boy says, "I don't believe in God. I believe in Russia — and her armed forces."

"Kids understand everything," says yet another. Indeed they do.