A Chinese director hopes his documentary about the deaths of hundreds of children in the collapse of a shoddily built school during the Sichuan earthquake will provoke answers, but doesn't expect any soon.

The devastating May tremor in southwest China killed 80,000 people. Many were children who had been napping or were at their desks in schools that crumbled while other buildings stood firm.

China vowed to punish those responsible after aggrieved parents blamed their children's deaths on substandard construction stemming from corruption and greed. No prosecutions have been reported yet, and families have been pressured into dropping their complaints against local officials.

Who Killed Our Children?, a feature-length documentary by 39-year-old director Pan Jianlin, records the anguish of parents whose children were entombed within the ruins of the middle school of Muyu, a village in the hill country of Sichuan.

"People have contacted my relatives and friends and told them to put pressure on me to stop my work," said Pan, whose film challenges the official story, which focused mainly on the heroism of soldiers who rushed to the rescue of quake victims.

Pan, detained for two days when he returned to Sichuan in June for more research, said he can't really answer the question posed in the film's title. He worries it will never be clear.

"I don't know how many children died, but I know that they died wastefully," said Pan. "I'm absolutely certain the government has an unshirkable responsibility."

Pan, who went to Sichuan to help with relief work like thousands of other civilians, said quake damage in Muyu was mild, but the school had been reduced to "just a pile of bricks."