The World Trade Center (or WTC) was a multi-building complex built in New York City on Earth in the 20th century. On September 11, 2001, the two main buildings (often referred to as the "Twin Towers") were destroyed in a highly coordinated terrorist attack involving hijacked airplanes.
No information beyond the image of the burning towers was given in the episode.
Early media reports on 9/11 suggested that Jeffrey Combs had died on one of the planes that hit the towers. Upon discovering this (a Jeffery Coombs had been one of the casualties, and was mistaken for the actor by news outlets), Combs publicly revealed that he was, in fact, still alive. Seth MacFarlane was scheduled to fly on one of the planes involved, but was unable to board due to arriving late.
According to liner notes included with Dennis McCarthy's Enterprise soundtrack release, the second day of recording for "Broken Bow" took place on September 11, 2001. McCarthy wrote, "I wish to dedicate this album to those who we lost on 9/11/01 and to those men and women who rose to the challenge and showed us that the human spirit will prevail no matter what."