The date is etched in blood in Utah and Mormon church history and, on a more intimate level, the family trees of people like Karen Maxwell, a mother of eight and choir teacher from Salt Lake City.

On Sept. 11, 1857, Mormon militiamen led the slaughter of 120 men, women and children on a wagon train bound for California in an incident known as the Mountain Meadows Massacre.

Chief among the instigators was Isaac Haight, a local militia and church leader. Several generations later, Karen Maxwell would come to know of him as her grandfather's grandfather.

For years, Mormon church officials downplayed the role Mormons played in the mass killing, first blaming Indians and then finding a scapegoat in church member John D. Lee, the only man executed for his role.

Now, a new book drawing on existing material and documents previously unavailable to scholars lays the blame largely on southern Utah church and militia leaders. They were otherwise good people, the authors say, who were caught up in the frenzy of the times and took up guns to try to cover up terrible mistakes.

The long-awaited release of "Massacre at Mountain Meadows" forces a re-examination of a dark episode for a faith community that puts families first. Mormons believe the family unit lasts for eternity, dutifully practice genealogy and cherish their pioneer past. So for descendants of massacre perpetrators, the book's gritty detail and naming of names can bring a painful reckoning.

Some are learning for the first time about their ancestors' culpability. Others, like Maxwell, are learning new details. She knew of Haight's role but didn't know his wife took in 17 young emigrant children who were spared. She also discovered that another ancestor refused to take part.

"It's important to know you had an ancestor who was out there," Maxwell said. "It personalizes it and brings it home and makes you ask yourself some of the important questions. What would I have done? If my ancestors did that, am I capable of doing this? We need to realize how something like this can come about."

The new volume is the work not of Mormon critics but former or current employees of the Church of Jesus Christ of Latter-day Saints, or Mormon church. Among them is assistant church historian Richard Turley, who said church leaders fully endorsed the "no-holds-barred, let-the-chips-fall as they may" approach.

"Our feeling was we just had to face it head on, let people deal with the truth and not with mythology," Turley said. "I know it's very uncomfortable, and I've gotten a lot of hate mail. People have said, 'Why peel off the scab?' It's because the wound is not healing. It needs major surgery."

Turley and co-authors Ronald Walker and Glen Leonard relied on hundreds of researchers rifling through archives nationwide, a specialist in 19th-century shorthand and newly available accounts from massacre participants from the archives of the First Presidency, the church's highest governing body.

The book tells the story in narrative form:

Utah was a tinderbox in September 1857. Concerned about the Mormon practice of polygamy (disavowed by the church in 1890) and a blooming theocracy out West, President James Buchanan dispatched federal troops to Utah Territory to put down a perceived rebellion. Mormons already driven from other states feared exile or death.

Mormon leaders girding for war ordered that no guns or grain be sold to emigrants, heightening tensions with California-bound settlers. Against that backdrop entered the Fancher-Baker wagon train, a group of mostly young families from Arkansas but also a few provocateurs who allegedly taunted the Mormons.

Judging the emigrants a threat, southern Utah Mormon leaders engineered what they hoped would appear to be an Indian attack on the wagon train at Mountain Meadows, a grazing spot.

A siege ensued. Amid some confusion and discord, a plan was hatched to lure the emigrants under a false flag of truce and kill all but those too young to "tell tales." The goal was to prevent news of Mormon complicity in the initial raid from reaching California and causing more hardship for the Mormons.

The militiamen led the attack, directing Paiute Indian allies in a plan "premeditated, evil and cunning."

"The massacre was not inevitable," the book concludes. "No easy absolution for the perpetrators is possible. ... The best that could be argued was that during a time of uncertainty and possible war, some of the Mormons, like other men and women throughout history, did not match their behavior with their ideals."

The book also delves into the psychology of group violence, concluding that all the ingredients were there, from an atmosphere of authority and obedience to stripping perceived enemies of their humanity.

"From the church's point of view, (the book) doesn't try to justify the motives of the people who did the killing," said Mormon historian Richard Bushman. "It's always been the tendency in the past to say it was a drastic action, but consider all they've been through. This book doesn't do that. There is nothing this party did or didn't do to justify the treatment they received. In that sense, it's an advance."

The book refutes claims that church president Brigham Young ordered the massacre. While Young's war rhetoric contributed to the volatile climate, finger-pointing to fix blame among local leaders afterward undermines the theory, the authors argue. An appendix implicates 68 militiamen.

"Most of the characters have been known," said co-author Walker, a retired Brigham Young University history professor. "Whether families have fessed up to it is unknown."

Eric Wadsworth was 17 when his great-grandmother said that her grandfather was a good man "who happened to be involved in something that went wrong" and was unjustly executed.

That man was John D. Lee, an energetic if quarrelsome Mormon convert who claimed heavenly visions and helped settle southern Utah at Young's request. Lee led the Indians on the initial attack and confessed to killing five or six emigrants.

Wadsworth, a 37-year-old software engineer, welcomed the new scrutiny of Lee, who had 19 wives and can claim tens of thousands of descendants. Lee was executed 20 years after the massacre.

"People should be mature enough not to get angry over history like this, especially when it is being presented in a non-biased way," Wadsworth said. "If this is used as a weapon, it'd be different."

Robert Briggs, a Fullerton, Calif., attorney and descendant of massacre participant Samuel Knight, said younger generations are better suited to process what happened.

"Now that 150 years have gone by, we ought to be able to look at this and simply accept it," Briggs said. "I look at it and I'm horrified by it. But I don't perceive it as some sort of stain on me or my family."

"If they made a mistake, fine, they'll pay for it one way or another," said another descendant of massacre participants, 65-year-old Keith Seegmiller of Cedar City, Utah. "We cannot comprehend the situations they were in, the pressures they were under. I do feel sorry for the ones who were killed. But there is nothing we can do about it now. We ought to let the whole thing rest."