A United Airlines Boeing 767-200 is seen about to one crash into one of the World Trade Center towers in New York, in this Sept. 11, 2001 photo. Knife-wielding hijackers crashed two Boeing 76s into the World Trade Center, toppling its twin 110-story towers. The Sept. 11 attacks may have cost the U.S. travel industry more than $200 billion, according to researchers at the University of Southern California. Credit: AP Photo/Carmen Taylor/FILE
