"A group of men -- variously described as Southern Paiute Indians, Mormons dressed as Natives, or a combination of Natives and Mormons -- deceived and attacked a group of 137 pioneers. Their wagon train was traveling from Arkansas, through Utah, and on to California. There are allegations that Mormons in the Mountain Meadows area created unrest among the Native population by spreading a rumor that the the pioneers were planning go to California and return with an army to attack the Natives and Mormons.
Apparently, many people on both sides died in the initial conflict. The pioneers then surrendered. Under a flag of truce, they were disarmed, and then slaughtered in cold blood. In all, 120 men, women and children of the wagon train were killed. 17 children under the age of 10 were considered "too young to tell," and were spared. Brevet Jamor J.H. Carleton noted in his investigation of the tragedy "that about one third of the skulls were shot through with bullets and about one third seem to be broken with stones." 4
There was "a popular and widespread impression that John D. Lee was the leader and arch criminal of the massacre." 5 He was made the scapegoat, tried twice and executed in 1877. There are allegations that the massacre was perpetrated by an underground Danite group. This theory appears to be a hoax since no such group existed in Utah at the time.
Brigham Young led a church cover-up, saying that the Natives were responsible for the massacre. He wrote that pioneers had caused the death of Natives by giving them poisoned meat, and by poisoning some of their wells."
Granted, I don't know all the facts, or how acurate the account is. And I'm sure agression on both sides fuled it, but it's still quite a bloody attack.