floors 4–9 suffered partial collapse and/or fire damage as a result of the september 11 attacks. floors 1–3 were not damaged. the building's structural integrity on its upper floors were partially compromised due to the impact of steel and other debris from the north tower. other collapsed sections were due to fire damage. portions of internal collapse and burnout were found on upper floors, mainly floors 6–8. the black exterior facade suffered severe fire damage. floors 5–9 were on fire after the collapse of the south tower. a section of the fuselage from united airlines flight 175 landed on the roof. 5 wtc was the least damaged building of the entire complex. the borders bookstore was undamaged after both towers collapsed. the building was the location of the survivors' staircase, which was recovered from the building's underground after it was demolished.