BAGHDAD, Oct. 2 — It started out as a family errand: Ahmed Haithem Ahmed was driving his mother, Mohassin, to pick up his father from the hospital where he worked as a pathologist. As they approached Nisour Square at midday on Sept. 16, they did not know that a bomb had gone off nearby or that a convoy of four armored vehicles carrying Blackwater guards armed with automatic rifles was approaching.

Moments later a bullet tore through Mr. Ahmed’s head, he slumped, and the car rolled forward. Then Blackwater guards responded with a barrage of gunfire and explosive weapons, leaving 17 dead and 24 wounded — a higher toll than previously thought, according to Iraqi investigators.