On September 11, 1857, members of the Mormon militia in southern Utah, with Paiute allies, killed most of the Baker-Fancher party, an emigrant wagon train from Arkansas on its way to California. Around 120 men, women and children died. Seventeen children, all under seven, were taken into local families and survived.
A siege came first. It began on September 7. The emigrants had corralled their wagons and dug in, and after several days of shooting that killed a number of them and wounded more, the militia sent a man in with a white flag and offered safe passage out under escort. The emigrants gave up their weapons and were divided into groups, and then, on a signal, they were killed by the men walking beside them.
The immediate context is the Utah War. President Buchanan had ordered several thousand troops to Utah to replace Brigham Young as territorial governor, and Johnston's Army was on the road. Martial law had been declared in the territory, the territory was arming, and word was reaching the south that a federal column was coming to destroy the settlements. The emigrants had also been behaving badly: several accounts, from sources hostile to them, describe arson, threats and poisoning of a spring and cattle. Emigrant parties passing through had been turned away from buying grain.
What happened next is why the case was never simply closed. John D. Lee led the attack. Lee said he had orders, and turned himself in on that basis in 1874. He was tried twice. Convicted at the second trial in 1876, he was executed by firing squad at Mountain Meadows in March 1877. No one above him was ever charged.
The Latter-day Saint church acknowledged the massacre and expressed regret in September 2007, on the 150th anniversary, at a ceremony at the site. Historians still argue about how much Brigham Young knew and when, and the surviving evidence has been read both ways. What is not disputed is that the killings were carried out by settlers, that children were deliberately spared while everyone else was not, and that the cover-up lasted a generation.