That didn't answer the question. Were people expecting the shooter to shout, "Cooee, I'm up here and going to start shooting, so for those who brought a gun and to the police, this is where you need to shoot".
Or did the shooting start and everyone dived for cover?
If the vast majority were disarmed, objective achieved. Now just to scrap the 2nd and start checking who's deemed responsible enough to own a gun. Then ban guns from public places, get the guns locked up when not in use, and most importantly, start teaching citizens a safe gun culture.