WASHINGTON — The witness wore a suit with no tie, the top button of his gray shirt undone. He had told this story many times, and now that he was in the United States, telling his story at last to a jury, he appeared neither hurried nor anxious.

Sarhan Deab Abdul Moniem, the witness, was a traffic officer that day in September 2007, when a convoy of Blackwater Worldwide trucks pulled into his traffic circle in Baghdad and started shooting. He held up two hands, showing jurors how he had pleaded with the American security contractors to stop. Through an interpreter, he spoke in a matter-of-fact way about running toward a victim inside a white Kia sedan.

“There was a lady. She was screaming and weeping about her son and asking for help,” Mr. Moniem said. He showed jurors how she had cradled her dead son’s head on her shoulder. “I asked her to open up the door so I could help her. But she was paying attention only to her son.”

More than four dozen Iraqi citizens like Mr. Moniem are scheduled to travel to Washington in the coming months to testify against the Americans who they say fired wildly on unarmed citizens, leaving 17 Iraqis dead. For years, they have waited as the case wound its way through the American court system. In a courtroom steps away from the Capitol, they are finally having their say. The Justice Department says it will be the largest number of foreign witnesses to testify in a criminal trial.