I frankly, think there might have been more then one shooter.
Three minutes is a LOT of time when someone is shooting unarmed people.
My Army training included an exercise demonstrating the power of ambush. My team of a half dozen ambushers took on two vehicles carrying unprepared soldiers. None of them was even able to return fire and probably none would have survived an actual attack. It is EXACTLY like shooting fish in a barrel.
Our mock attack lasted about fifteen seconds, just long enough to empty a twenty round magazine with aimed fire.
Get a clock with a second hand and see how many times you can say "bang" in three minutes. Even if you allow a full two seconds between shots, that is still 90 shots. You could kill ten, wound forty, and still miss almost half the time.
I think it was a terrorist attack and political correctness rules again and it will never be told.
The guy that lugged the grenade in a tent at war time overseas years ago shows they have infiltrated our military. It was reported years ago that due to GOOD interrogation and manuels being uncovered; the terrorist call handbook cites infiltration everywhere in the U.S. blending in with and being American.