On September 11, a bunch of violent attackers carried out a brutal massacre of innocent Americans in the name of their religion, one which sanctioned polygamy and often saw non-members as enemies. The religion was founded by a self-proclaimed prophet who claimed to have been given divinely revealed scripture, who freely engaged in polygamy, and who was not always above using violent means to achieve his goals. After his death, most of his followers fell in behind a new leader, but others did not, thus creating a separate sect that at times experienced tension with the majority.
Yes, we remember what happened in 2001, but that was the second time that the above paragraph became true. The first time came about many years earlier, and is remembered as the Mountain Meadows Massacre. On September 11, 1857, after besieging a party of emigrants from Arkansas for four days, a militia consisting of Mormons, with the aid of some local Paiute Indians, killed most of the emigrants, allowing some children among them to survive. In the aftermath and over time, the Mormons eschewed both violence and polygamy, making such behavior from them today very far-fetched indeed. In this sense, they have become very different from those who carried out this century's 9/11.
Read more about Mountain Meadows here, here, here and here. Four years ago, I was in southwestern Utah and visited the memorial at Mountain Meadows. From a viewing area, the site may be seen, including the memorial itself.