Their has been a confession found where Brigham Young admitted to the Mountain Medows MAssacare. For those who aren't or weren't part of the morg this is a bit of history
120 settlers from caliofornia were killed by Natives under the direction of John Lee. This was a time fo great worry, early moromns were already forced violently out of the East ,then Kirkland Ohio and then Navouh Illionis. The early "prophets" of the church founded both kirkland and Navouh as safe havens and it was deeply disappointing that they failed at these locations. Utah looked sucessful but Buchanan who was the presdient at the time was threating to replace Brigham Young with a non mormon Andrew Cumming. He sent in martial forces w/o telling Brigham what was going on .About this time 120 emmigrants heading to California stopped in Utah for supplies , the morg was reading for a seige and didnt have much to spare. There has been controversy to this day about how much Brigham knew , now thanks to these documentts it seems everything .