For more than a month after the killings, Sgt. Lemuel Lemus stuck to his story. “Proper escalation of force was used,” he told an investigator, describing how members of his unit shot and killed three Iraqi prisoners who had lashed out at their captors and tried to escape after a raid northwest of Baghdad on May 9. Then, on June 15, Sergeant Lemus offered a new and much darker account. In a lengthy sworn statement, he said he had witnessed a deliberate plot by his fellow soldiers to kill the three handcuffed Iraqis and a cover-up in which one soldier...