At the point of the attack, the two suspects apparently drove up and opened fire upon an unarmed security guard who was accompanied by a 60-year-old Garland police officer. The unarmed guard was struck the volley of gunfire. The veteran Garland officer then drew his duty-issue Glock pistol and opened fire on the suspects.
The officer killed one terrorist and wounded the other in his initial volley of return fire. Witnesses claim there was a brief pause, and then the officer fire two more shots to kill the still-moving terrorist as he appears to be reaching for a backpack. The entire event lasted 15 seconds, with heavily-armed Garland SWAT converging on the scene immediately afterward.
While every pistol is different from another in its ejection pattern, and the movement of the officer and the cant of his gun precludes us from knowing exactly where he was, there, is a distinct trial of shells showing that the officer was moving forward from the bottom left of the photo above towards the terrorists at the rear of the vehicle. He appears to have opened fire from 20 yards away, and fired at least a dozen shots by the time he reached an area near the traffic cones, roughly 7-10 yards from where the terrorists died.Some more information and photos at the link.
(H/t Bayou Renaissance Man)