For the firefighters stationed near the World Trade Center, the call of a fire in Tower One was not unusual; in fact, they responded to fire alarms at the towers on a daily basis. However, the extent of the fire on September 11, 2001, was far beyond ordinary. Entire floors were on fire. And within twenty minutes, both towers were on fire. The buildings' design—with lots of open space on each floor and no interior support columns—allowed the fires to burn freely with no firewalls to slow their progress. Observers reported that the smoke from the fires was black, indicating that the fire was provided with lots of fuel. The smoke turned white as it rose into the air, which meant the fire was intensely hot. Of course, no one realized at the time that each crash's impact blew off the fire-retardant...