At the time, the American military had sent advisers to Uganda to help plan an attack on the headquarters of the Lord’s Resistance Army in Garamba National Park, in northeastern Congo. Ugandan helicopters strafed the camp, narrowly missing Joseph Kony, the group’s leader, who has been indicted by the International Criminal Court on crimes against humanity. Afterward, the fighters scattered and vented their outrage on nearby villagers.

This time, the L.R.A. seemed to have had a different strategy.

Instead of storming into villages and burning down huts (as it did in 2008 and early 2009), the group sent in relatively clean-cut soldiers dressed in proper military fatigues.

“They came in saying they were the national army and they wanted to know where were the churches and schools, so they could protect them,” said the Rev. Joseph Nzala, a priest in Tapili.

Eastern Congo has been a dumping ground for various armed groups for years, so it is not surprising that the villagers might have been confused. But as soon as they gathered, the roughly two dozen fighters roped them up at gunpoint and took them away. The band repeated the ruse in village after village, steadily expanding but eliminating hundreds along the way.

There were no peacekeepers or real government soldiers around, and when the killing started Dec. 14, all the people could do was run. Several men who escaped said the fighters must have had their own secretive selection process because there was no way of knowing who was about to die.

“Was it someone walking slow or someone old? No,” said Charles Emabe, who managed to slip away one night.