We now have more information on the questions I discussed below. We now appear to have a better idea of where the shooter was and how Secret Service agents were able to shoot and kill the shooter only a few seconds after he opened fire. It appears the distance between the rooftop and the stage was more like 150 yards. The building in question was a large single story building just outside the security perimeter of the event. Much closer and just to the side of the stage was another building, with a higher roof. On the roof, with a commanding over the whole area was a Secret Service counter-sniper team. It appears that some onlookers noticed that gunman on the roof and began trying to alert police and/or Secret Service agents on the ground. At this point the counter-snipers either saw the gunman on their own or perhaps heard warnings about someone on the roof in their earpieces.
So it appears they were already looking to see what was going on before the gunman fired his first shots. Quite possible this was only a matter of a few seconds. But this helps explain why they were able to shoot the gunman dead only a few seconds after he opened fire. If you see the map of the buildings in question the counter-snipers would have had a direct and unobstructed line of sight toward the shooter.
