 They believe that the gunman might have used an air conditioning unit to get on top of the roof. Well, we know he did buy a ladder at Home Depot that morning, and we're not quite sure where that ladder was. By the time other officers came for backup, the gunman had climbed on top of the building, was positioned above and behind the snipers inside the building. Two other officers who heard the snipers' call came and tried to get on the roof, but then it was too late.