The Sunni militants who seized the riverside town of Buhriz late last month stayed for several hours.

The next morning, after the Sunnis had left, Iraqi security forces and dozens of Shiite militia fighters arrived and marched from home to home in search of insurgents and sympathizers in the rural community, dotted by date palms and orange groves.

According to accounts by Shiite tribal leaders, two eyewitnesses and politicians, what happened next was brutal.