The Official Account
On 9/11, the Twin Towers came down because of damage produced by the impact of the planes combined with fires ignited by the jet fuel. After burning for 101 and 56 minutes, respectively, the north and south towers came down rapidly but without the aid of explosives. 
The Best Evidence
The Twin Towers were built  to withstand the impacts of airliners having approximately the size and speed of those that struck them. And office fires, even if fed by jet fuel (which is essentially kerosene), could not have weakened  the steel structure of these buildings sufficiently to collapse as suddenly as they did.
Only the top sections of these buildings were damaged by the impacts and the resulting fires, whereas their steel structures, much heavier towards the base, were like pyramids  in terms of strength. So the official account, which ruled out explosives, cannot explain why these buildings completely collapsed.
References for Twin Towers Point 4
- “NIST found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001.” NIST NCSTAR 1, Final Report on the Collapse of the World Trade Center Towers, September 2005, p. xxxviii.
- Federal Emergency Management Agency, (2002), World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations.
“Towers collapse shocks engineers,” MedServ, 9/11/01. “
Twin Towers Engineered To Withstand Jet Collision,” Seattle Times, February 2, 1993.
- “Twin Tower Fires Not Hot Enough to Melt or Weaken Steel!” YouTube: 9/11 Truth Videos
- Architect Mario Salvadori explains: “The load on the columns increases with the number of floors of the building, and their weight must vary in the same proportion.” (Dr. Mario Salvadori, “Why Buildings Stand Up,” New York: W.W. Norton, 1980, p. 117). The lower the floors, the stronger the steel structures. So even if the impacts and fires had caused the top sections of these buildings to collapse, the collapses would have been arrested by the lower floors.