Airliner crashes and resulting fires fueled by kerosene/jet fuel and office materials as government and media contend.
Thermite/thermate/nano thermite placed throughout the buildings as advocated by Professor Steven Jones, Kevin Ryan and Neils Harrit.
Big, bottom-up nuclear explosion as Dmitri Khalezov advocates.
Mini-nukes in a sequence of nuclear explosions as advocated by the anonymous physicist and Dr. Ed Ward, M.D.
Directed Energy Weapons as advocated by Dr. Judy Wood (and Morgan Reynolds).
All of the above.
None of the above.