It was stated that one of the bullets was definitely a kill shot and two were not fatal. They also stated that one of the bullets was fired from within 1-2 feet away. It should have been obvious from this information and the angle of entry what had happened almost from the start. This is not the type of mistake NCIS would have made. It should also be obvious from where the kill shot originated. Close or far.
In the final scene between Abby and McGee, the wide shot of McGee clearly shows his camera marker painted on the floor, and McGee then glances down at it to confirm his position.