How he got on top of that building is the question not being answered. A freeper on another thread said a ladder was left in the bush behind the building. That means it was all put in place before he got there. Was the rifle placed there as well, or did he walk into an area whose ingress and egress was supposed to be controlled, carrying a rifle? He wouldn't have likely carried a ladder as well. Was the ladder attached to the building already, or was it left on the ground for him to use?
There are many puzzling questions, that should have already been addressed. The longer it takes for simple questions like this to be answered by authorities, the more believable that this was an inside job.
Normally metal buildings don’t need a ladder to the roof unless there’s HVAC equipment.
I don’t see any in the photo.