The important point is that during Stage 2, the collapse was accelerating at 32 ft/sec^2, i.e., it was essentially in free fall.
The 9/11 truthers claim that this could happen only if the building was deliberately demolished (i.e., it requires simultaneous catastrophic failure of structural members all over the building, and therefore the free fall indicates that the building fell for some other reason than being damaged by falling debris from the main towers and the fires that raged uncontrolled for six hours.
This article on implosionworld.com by a building demolition specialist says that a collapse due to structural damage is consistent with all the known facts.
The truthers also claim that the NIST did not do the analysis summarized in the graph above, until pressed by them, and that this is a sign of a cover-up.
Your mileage may vary.
The text from the NIST report states the following:
3.6 TIMING OF COLLAPSE INITIATION AND PROGRESSION
The timing of global collapse of WTC 7, as indicated by downward motion of the north exterior face, was investigated using a video of the collapse taken from the vantage point of West Street near Harrison Street (Camera No. 3, Figure 5-183 of NIST NCSTAR 1-9). An initial analysis compared the observed time it took for the roofline to fall approximately 18 stories to the free fall time under the force of gravity. A more detailed analysis examined the vertical displacement, velocity, and acceleration through different stages of the collapse process. (NIST NCSTAR 1-9, Chapter 12)
The time that the roofline took to fall 18 stories or 73.8 m (242 ft) was approximately 5.4 s. The theoretical time for free fall (i.e., at gravitational acceleration) was computed from
where t = time, s; h = distance, m (ft); and g = gravitational acceleration, 9.81 m/s2 (32.2 ft/s2). This time was approximately 3.9 s. Thus, the average time for the upper 18 stories to collapse, based on video evidence, was approximately 40 percent longer than the computed free fall time.
A more detailed examination of the same video led to a better understanding of the vertical motion of the building in the first several seconds of descent. NIST tracked the downward displacement of a point near the center of the roofline, fitting the data using a smooth function. (The time at which motion of the roofline was first perceived was taken as time zero.) The fitted displacement function was then differentiated to estimate the downward velocity as a function of time, shown as a solid curve in Figure 3- 15. Velocity data points (solid circles) were also determined from the displacement data using a central difference approximation. The slope of the velocity curve is approximately constant between about 1.75 s and 4.0 s, and a good straight line fit to the points in this range (open-circles in Figure 3-15) allowed estimation of a constant downward acceleration during this time interval. This acceleration was 32.2 ft/s2 (9.81 m/s2), equivalent to the acceleration of gravity g.
For discussion purposes, three stages were defined, as denoted in Figure 3-15:
• In Stage 1, the descent was slow and the acceleration was less than that of gravity. This stage corresponds to the initial buckling of the exterior columns in the lower stories of the north face. By 1.75 s, the north face had descended approximately 2.2 m (7 ft).
• In Stage 2, the north face descended at gravitational acceleration, as the buckled columns provided negligible support to the upper portion of the north face. This free fall drop continued for approximately 8 stories or 32.0 m (105 ft), the distance traveled between times t = 1.75 s and t = 4.0 s.
• In Stage 3, the acceleration decreased somewhat as the upper portion of the north face encountered increased resistance from the collapsed structure and the debris pile below. Between 4.0 s and 5.4 s, the north face corner fell an additional 39.6 m (130 ft).
As noted above, the collapse time was approximately 40 percent longer than that of free fall for the first 18 stories of descent. The detailed analysis shows that this increase in time is due primarily to Stage 1. The three stages of collapse progression described above are consistent with the results of the global collapse analyses discussed in Chapter 12 of NIST NCSTAR 1-9.