The Mountain Meadows massacre of 1857 is considered the worst mass killing in U.S. history. On Sept. 11, 1857, 147 pioneers from Arkansas were killed in the territory of Utah by a raiding party whose ties to the church remain in dispute to this day.
The territorial governor and president of the church, Brigham Young (played by Terence Stamp), is heard -- in a voice-over -- encouraging vengeance. "Blood atonement" he called it.
The suggestion is that Mormons were seeking revenge after they were persecuted and driven out of Missouri. It happened at a time when Washington was threatening to take over the territory and Young wanted nothing of it.
Michael Purdy, a spokesman for the church, disputes historical claims that Young ordered the wagon train of settlers slaughtered to keep non-Mormons out of Utah.
"While no one fully knows what happened at Mountain Meadows 150 years ago, we recognize it was a terrible tragedy for all," he told ABCNEWS.com. "The church has done much to remember those who lost their lives there."
At a memorial service and dedication of the Mountain Meadows monument near Cedar City, Utah, in 1999, church president Gordon B. Hinckley said, "I sit in the chair that Brigham Young occupied as president of the church at the time of the tragedy. There is no question in my mind that he was opposed to what happened. Had there been a faster means of communication, it never would have happened."
Explaining the reference to "faster means of communication," Purdy said Young "sent a messenger by horseback but the messenger did not arrive in time to prevent the tragedy."
Brian Patrick, a film studies professor at the University of Utah who produced the documentary "Burying the Past," which may be the genesis of the "September Dawn," disagreed.
"Relatives of the Arkansas victims say Young ordered the killings," Patrick told ABC News. "No one knows for sure, but we do know after reviewing all the records that at the very least Young covered up a serious, serious crime."
Retired NFL star quarterback Steve Young, who is the great-great-great-grandson of Brigham Young, declined to comment, as did presidential candidate Romney.
To Patrick, the massacre evoked an eerie resonance. Referring to the Sept.11 attack on the World Trade Center, he said there's a "direct correlation with what happened at Mountain Meadows. It's the notion of religion running amok and religious fanaticism both then and now."
But Patrick and others stopped short of predicting any lasting damage to the church. "I don't think this, the movie or the PBS portrayal of the massacre will take them out," he said.
In Utah, where about two-thirds of the state's population is Mormon, reaction seems to depend on one's age and commitment to the church.
A young Salt Lake City computer technician brushed off the massacre controversy with a simple explanation. "It was a failure to communicate," he told ABC News. "There's nothing more sinister to it than that. It's not a big deal."
An architect from Park City who left the church years ago was harsher. "We were never, ever taught about the Mountain Meadows massacre in school," he said. "And you can and should quote me on that."
Historic amnesia is something a lot of people talked about when referring to church history and its impact on the LDS church today. Both the massacre and the early days of polygamy, which has long been outlawed by the church, are particularly painful to the faithful.