On September 11th, 2001, the 28th anniversary of the CIA-directed military coup d'etat in Chile, terrorists (but not Arab terrorists) took control of four planes and (according to the official story) crashed two of them into the World Trade Center towers, causing fires within. Again according to the official story (pre-written and rushed into print in the mainstream media immediately after the events, together with the identity of the alleged culprit) the fires then caused the steel girders to melt and the towers to collapse.

 http://www.indyhawaii.org/news/2002/12/306_comment.php

 http://www.serendipity.li/wtc.html#what_actually_happened