In Mountain Meadows, Utah on September 7, 1857.Aaargh! Apologies to all ... I mistyped and got the date wrong. Based on the comments so far I think that my brother and Rick Lugari are clued in on this incident.
Answer (in invisi-type)
The Fancher party, comprising more than 120 emigrants from Arkansas headed to California by wagon train, was attacked by a party variously described as Paiute Indians acting at the behest of Mormons, or Mormons disguised as Indians, or a combination of Mormons and Indians. The emigrants were besieged for five days; then a group of Mormons promised to lead them to safety. Instead, on September 11, they shot all but seventeen of the emigrants point-blank in what is now called the Mountain Meadows Massacre. The seventeen survivors, all children under five, were distributed among Mormon families. All but one were later recovered by a federal agent.