Among the questions still to be answered, the official said, is whether at any time nearby Iraqi security forces began firing, possibly leading the Blackwater convoy to believe it was under attack and therefore justified in returning fire. It is also possible that as the car kept rolling toward the intersection, the Blackwater guards believed it posed a threat and intensified their shooting.

Blackwater has said that its guards were fired upon and responded appropriately.

Witnesses close to the places where most of the Iraqi civilians were killed directly facing the Blackwater convoy on the southern rim of the square all give a relatively consistent picture of how events began and unfolded.

The Blackwater convoy was in the square to control traffic for a second convoy that was approaching from the south. The second convoy was bringing diplomats who had been evacuated from a meeting after a bomb went off near the compound where the meeting was taking place. That convoy had not arrived at the square by the time the shooting started.

The events in the square began with a short burst of bullets that witnesses described as unprovoked. A traffic policeman standing at the edge of the square, Sarhan Thiab, saw that a young man in a car had been hit. In the line of traffic, that car was the third vehicle from the intersection where the convoy had positioned itself.

“We tried to help him,” Mr. Thiab said. “I saw the left side of his head was destroyed and his mother was crying out: ‘My son, my son. Help me, help me.’”

Another traffic policeman rushed to the driver’s side to try to get her son out of the car, but the car was still rolling forward because her son had lost control, according to a taxi driver close by who gave his name as Abu Mariam (“father of Mariam”).

Image Haider and Mariam Ahmed lost their mother and older brother in the shooting involving guards for Blackwater USA. Credit... Johan Spanner for The New York Times

Then Blackwater guards opened fire with a barrage of bullets, according to the police and numerous witnesses. Mr. Ahmed’s father later counted 40 bullet holes in the car. His mother, Mohassin Kadhim, appears to have been shot to death as she cradled her son in her arms. Moments later the car caught fire after the Blackwater guards fired a type of grenade into the vehicle.