Then the guard would see the people leave
Maybe, if the attack was imminent.
The real attack could have been scheduled for hours later, or the next day.
I am thinking of the terrorist attack on that mall in Europe, a few years back.
I understand the rifles were scattered around the room, NOT, stacked and ready near one of the two sniper platforms/broken windows.
Then we have the six minute pause before he sprays the crowd with bullets.
Tells me the loser was in prep mode, not waiting in position to start an attack
Hopefully, we find out what happened.
Importantly, Homeland Security figures out how to defend agaist this kind of attack