What explanation could the hapless Jiang give them? Local party officials in China are squeezed between maintaining order and some sort of minimal popularity, while saddled with Beijing's "unfunded mandates" - required to provide social services at the local level with no revenue grants from the centre. So they are famously rapacious, collecting random taxes, making sweetheart deals with developers and skimming any contract in their district - not just to line their own pockets, though that happens a lot, but simply to make local revenue equal local expenditure. When they have to build schools on skimpy budgets, the temptation to cut corners on costs - tofu construction - is strong. Ultimately, 7,000 children paid the price.

When HBO documentary supremo Sheila Nevins heard a moving, on-the-ground NPR broadcast from a reporting team that had been in Sichuan province for other reasons, Alpert and O'Neill were dispatched to Chengdu. They got there nine days after the quake. They filmed lots of ruins, "but there was no documentary thread. I thought we were finished, and packed up," explains Alpert.

But the next day they drove (literally) into the film's most riveting scene. Several hundred parents were striding along with banners calling for justice and carrying black-shrouded photos of their dead children. "We hopped out of the car and just started filming," says Alpert.