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 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 U.S. security contractors to stop. Through a translator, 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,” 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 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 U.S. court system. In a courtroom steps away from the Capitol, they are finally having their say. The U.S. Justice Department says it will be the largest number of foreign witnesses to testify in a criminal trial.

“Significant resources have been expended to ensure the witnesses have access to the U.S. court system,” said Andrew C. Ames, a spokesman for the FBI, which is coordinating the travel arrangements.

Family members and survivors have said they see this trial as a test of America’s judicial system. And though they have expressed frustration and skepticism, dozens have volunteered to take part.

When the Justice Department indicted five former Blackwater guards in 2008 and reached a plea deal with a sixth, prosecutors said it was message that, whether in a war zone or not, nobody was above the law.

But the case has suffered repeated setbacks, frequently of the government’s own making. In Iraq, the delays contributed to the impression that Blackwater operated with impunity. Prosecutors ultimately dropped charges against one guard, citing a lack of evidence, and have gone to trial against the remaining four: Dustin L. Heard, Evan S. Liberty, Nicholas A. Slatten and Paul A. Slough.

Nobody disputes that a team of Blackwater guards, working for the U.S. State Department, drove four armored trucks into a busy traffic circle and opened fire. It is clear that it all began with the white Kia, and that by the time the shooting subsided, there were many dead. But the four former security guards standing trial – three on manslaughter charges, one on a murder charge – say they believed they were being ambushed in the traffic circle. A car bomb had just detonated a short distance away, and the white Kia, defense lawyers argue, looked like a potential follow-up bomb. The guards also say they were under fire from insurgents.