The Official Account
The Twin Towers were destroyed by three and only three causes: the impacts of the airliners, the resulting fires, and gravity. 
The Best Evidence
During the destruction of the Twin Towers, huge sections of the perimeter steel columns, weighing many tons, were ejected horizontally as far as 500 to 600 feet, as seen in multiple photographs  and maps. 
These high-speed ejections of heavy structural members cannot be explained by the fires, the pull of gravity, or the airplane impacts (which had occurred about an hour earlier).
Human bone fragments  approximately 1 cm long were found in abundance on the roof of the Deutsche Bank following the Towers’ destruction, which further points to the use of explosives. Pancaking or tamping of floors from above would tend to trap bodies, not hurl splintered bones over 500 feet horizontally.
References for Twin Towers Point 2
- NIST NCSTAR 1, Final Report on the Collapse of the World Trade Center Towers, September 2005, 144-45 (pdf: 194-5).
- The Scientists for 9/11 Truth website shows a photo of “Impaled Steel Columns at the 20th Floor of the World Financial Center Building 3 (WFC3).”
See also video one and video two showing horizontal ejections.
- In addition to WFC3 (American Express Building), the FEMA Report, “7. Peripheral Buildings”, shows that similar debris hit the Winter Gardens, 500-600 feet distant, and includes a map showing the location of these buildings.
- “Remains bring hope, frustration for 9/11 families,” USA Today, April 20, 2006.