The North Tower burns
There were around 20,000 gallons of jet fuel in the Boeing 767 that hit the North Tower. That building had 110 stories. What slowed its collapse was the higher impact point of the plane, the 80th floor. That strike killed hundreds immediately, but the subsequent burn would kill even more. The people above the 80th floor had no way down, no way out.
Tenants stranded in windows
The people in the impact zone of the planes suffered first. Of those people in the south tower, only 14 escaped. Many became trapped on the floors with nowhere to escape. With rising heat and no chance of a rescue, many resorted to the unthinkable. They threw themselves or fell trying to climb the building.