They teach about that in Church History. It is not a secret, the guy responsible was arrested by a Mormon Sheriff, tried by a Mormon prosecutor before a Mormon Judge and Mormon Jury. He was convicted and went to jail.
A lot more is made of this than it deserves. There were other murders that happened in that time frame, but you never hear about them.
People try to blame Brigham Young for this event, but he had sent word to let these people pass through. The message arrived after the event.
More to the point, and precisely relevant to this thread, would be to say the guy was arrested by a duly elected Sheriff who was a Mormon also, prosecuted by a man who happened to also be a Mormon, before a Constitutionally authorized Judge who was also a Mormon, and convicted by a Constitutionally paneled Jury who were also Mormons, and he was sent to jail by a jury of his peers as is specified in the Constitution of the United States, a Constitution upheld by people who also happened to be Mormons. [If Mitt Romney is elected to be President, he will also be Mormon but he will serve by and for the Constitutional authority, not because he is also a Mormon. Frankly, I can live with that nicely, thank you.] (^ :