In the North Tower the plane severed all three stairwells, leaving them impassable from the 92nd floor up. Hundreds of people were killed instantly; hundreds more were alive but trapped. The 911 system was overwhelmed, and operators, who did not know rooftop rescue was impossible, told many callers to stay where they were. [1]
In the South Tower one stairwell, A, stayed passable. At least four people came down it from the 81st floor or above. Many others in and above the impact zone went up, hoping to be rescued from the roof, which was locked. [1]
Twelve firefighters, a Port Authority officer and three civilians who were in stairwell B of the North Tower when it collapsed survived. [1]
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