After the shootings, American soldiers found plenty of empty bullet casings 7.62 millimeters in diameter. Had the 7.62-millimeter casings been from an AK-47 rifle, a common insurgent and Iraqi police weapon, they would have been 39 millimeters long. Had they been from a PKC machine gun, another common Iraqi weapon, they would have been 54 millimeters long. The soldiers did not find any of those, the military official said.

Instead, the official said, the casings were 51 millimeters long, the length used by NATO weapons, including the M-240 machine gun, a standard automatic weapon used by the America military and American security contractors, the official said. The soldiers also found empty 5.56-millimeter casings of the type used by the M-4 and M-16 rifles that American troops and contractors bear.

The F.B.I. has been interviewing soldiers from the unit that responded to the scene, the Third Battalion of the 82nd Field Artillery Regiment, which is part of the Second Brigade of the First Cavalry Division, to collect information in its investigation of the shooting, the official said.

Only one of the Kurdish witnesses, a guard who would give his name only as Sabah, saw the first shots fired by the Blackwater guards into a white sedan, killing a man and his mother and setting the events in motion.

Two others, Mr. Waso and his driver, Sirwan Ali, went to the roof after the shooting started and observed long enough to see the Blackwater vehicles leave the square. Eventually both went down to help the victims, they said. All three men have military backgrounds.

When asked if anything had occurred to provoke the initial shots from Blackwater, Sabah said: “Nothing at all. No mortars. No shooting.”

All he saw, Sabah said, was that the white sedan “moved a little bit and they started shooting.”

As events unfolded and the Blackwater guards unleashed a storm of gunfire into the crowded square, Mr. Waso and Mr. Ali both said, they could neither hear nor see any return fire. “It was one-sided shooting from one direction,” Mr. Waso said. “There wasn’t any return fire.”