For years, villagers who lived near the Alau dam in northeastern Nigeria had told government officials that the structure was broken and the reservoir behind it too full.

But in early September, after heavy rains, a half-dozen officials stood overlooking the brimming reservoir, their feet squelching in the mud as they tried to reassure Nigerians that the dam was in good condition.

“The dam is not broken,” Alhaji Bukar Tijani, the government official leading the delegation, said that day. “People should not be afraid.”

Four days later, water ripped through the Alau dam wall, leaving two-thirds of the city of Maiduguri underwater, killing up to 1,000 people, said rescue and security workers, and displacing nearly half a million.

After the disaster, government officials blamed God, climate change and the poorest people of Maiduguri, who they said had put themselves in harm’s way by living in cheap homes along the Ngadda River.

But in fact, government agencies knew the dam was badly damaged and did not fix it or correct operational mistakes despite repeated warnings, both from local residents and from engineers who spent six years studying the dam.