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This video tracks the motion of the NW corner of Building 7 of the World Trade Center on 9/11 2001. For a period of ~2.5 seconds. This means it was falling through itself for over 100 feet with zero resistance, an impossibility in any natural scenario. This period of freefall is solid evidence that explosives had to be used to bring the building down.
In the final draft for public comment (August 2008) NIST denied that WTC7 fell at freefall. In the final report in Nov 2008 they reversed themselves and admitted freefall, but denied its obvious significance.