The Official Account
The Twin Towers were brought down by airplane impacts, jet fuel, and office fires. 
The Best Evidence
Experience, based on physical observation and scientific knowledge, shows that office fires, even with the aid of jet fuel, could not have reached temperatures greater than 1,800 degrees Fahrenheit (1,000 degrees Celsius).
But multiple scientific reports  show that metals in the Twin Towers melted. These metals included steel, iron, and molybdenum – which normally do not melt until they reach 2,700°F (1482°C), 2,800°F (1538°C), and 4,753°F (2,623°C), respectively.
References for Twin Towers Point 1
- NIST NCSTAR 1, Final Report on the Collapse of the World Trade Center Towers, September 2005, p. 15. Regarding airplane impacts, see pp. 150-51; Jet fuel, pp. 24, 42; Fires, pp. 91, 127, 183.
- RJ Lee Group, “WTC Dust Signature Report: Composition and Morphology,” December 2003, p. 21.
RJ Lee Group, “Expert Report: WTC Dust Signature,” May 2004, p. 12.
Heather A. Lowers and Gregory P. Meeker, US Geological Survey, US Department of the Interior, “Particle Atlas of World Trade Center Dust,” 2005.
Steven E. Jones et al., “Extremely high temperatures during the World Trade Center destruction,” Journal of 9/11 Studies 19 (January 2008).
For discussion and summary, see David Ray Griffin, The Mysterious Collapse of World Trade Center 7, pp. 39-44.