When the people began coming out of the towers into the lower lobby, the NYC firefighters made a virtual wall so that the people coming down couldn't see what was happening in the area outside. They didn't want to start an all out panic.
One of the most amazing scenes is when the fire fighters are in the North Tower Lobby, and hear a 'ding'. An elevator door opens, and a group of people who had been in the elevator when the plane hit, walk out, looking around, not having a clue what had happened.