...that the attacks on the Pentagon and the WTC weren't the first religiously-based atrocities committed on this day in history? You'll have to go all the way back to 1857 for for the first terrible event.
If you are interested in learning more about the Mountain Meadows massacre, I suggest one of the following books:
American Massacre, by Sally Denton
Blood of the Prophets, by Will Bagley
Massacre at Mountain Meadows, by Ronald Walker and Richard Turley (this is the only book on the subject sanctioned by the LDS church)
If I had to suggest one book, it would be Bagley's. His is the only one that seems to be able to wrap itself around the often troubling religious aspects of the event.