The gunmen arrived on motorcycles and in the backs of Toyota pickups, brandishing Kalashnikovs and dressed in the telltale, sand-colored uniforms of Sudan’s Rapid Support Forces militia. Over three days they killed hundreds of men and teenage boys in a Darfur refugee camp, according to witnesses, local human-rights groups and international aid agencies.

The Sudanese-Arab militia worked its way systematically through the tents, shacks and mud houses of the Ardamata camp, shooting male residents. The settlement in western Darfur was home to about 30,000 non-Arab Sudanese people who had fled earlier bouts of fighting in the country’s civil war, which started nearly seven months ago.