Residents have long objected to the hulking garbage dump and the trash-soaked water that pours into narrow lanes of surrounding neighborhoods when it rains, leaving pools of standing water where mosquitoes breed.

Successive governments have groped for a solution, at one point proposing that city garbage be transported by rail and dumped in a quarry, said Raisa Wickrematunge, co-editor of Groundviews, a citizen journalism initiative. Still, year after year, the mountain of garbage continued to grow, with tractors scaling its sides to dump fresh loads.

“It requires urgent action, too, because as we have seen, people’s lives are at stake,” she said.

Harsha de Silva, Sri Lanka’s deputy foreign minister, described the collapse as the culmination of “a problem running for decades, perhaps as long as 20 years.” He expressed sympathy to bereaved families, but he said it was “unfortunate” that many families had been compensated to move from the area but failed to do so.

He added that garbage would no longer be deposited at the site.

The police said Saturday that they thought an explosion underneath the dump had set off the landslide, sweeping as many as 100 houses off their foundations and sending them crashing into neighboring homes. Some were deposited 30 yards from their original locations, a few landing on the roofs of other houses, said Priyantha Jayakody, deputy inspector general of police.

Mr. Jayakody said it was unclear whether the explosion had been natural or man-made.

Sugath Dammika, 41, a laborer, said he had just dropped off two of his sons at his sister-in-law’s house when it was buried by waste. As he spoke, his wife was at a government morgue, identifying the body of their elder son. The younger son is still missing.