In God’s name: Is it madness?

Tromping around Salt Lake City with a long beard, wild-man hair and wooden staff, the man was simply known as the “Jesus Guy.” But today Brian David Mitchell, and his wife, have a wider reputation as the alleged kidnappers of 14-year-old Elizabeth Smart. Dressed in white robes and a face-covering veil, Elizabeth was undetected for months -- “It wasn’t suspicious at all to see a girl in a veil,” said one acquaintance -- and became convinced that God had commanded her to become Mitchell’s second wife.

It’s all inexplicably biblical, unless you know something about Mormon Fundamentalists, as Mitchell and others cast out of the Church of Jesus Christ of Latter-day Saints for practicing polygamy call themselves. The split between the Fundamentalists and the official Mormon church is the backdrop for Jon Krakauer’s new book, “Under the Banner of Heaven,” in which he explores the fanatical fringe of Mormonism and the nexus between extremist faith and predatory violence through the story of a bone-chilling double murder committed in 1984 in the heart of Mormon country.

Krakauer is an adept chronicler of extremists, and he’s as intent on understanding religious fanatics as he was in his earlier books on exploring the obsessions of monomaniacal adventurers. The tour guide of choice for secular quests, he brings his skills to bear here on a look at the insular world of Mormon cast-outs. “Under the Banner of Heaven” is illuminating rather than sordid, more provocative than sensational.

On July 24, 1984, 24-year-old Brenda Lafferty and her 15-month-old daughter, Erica, were stabbed to death in their suburban home in Utah by Dan and Ron Lafferty, Brenda’s brothers-in-law and the uncles of Erica. At first, the setting seems an odd fit for Krakauer. The author of “Into the Wild,” the story of a young man whose self-imposed exile led to his death by starvation in the Alaskan wilderness, and “Into Thin Air,” the bestselling account of a 1996 expedition to Mt. Everest that killed eight climbers, he has written insightfully about the seductive call of the wilderness. As a self-professed agnostic with no ties to Mormonism, he doesn’t bring to this book the same visceral sense he did to his others. But if “Under the Banner of Heaven” is a less urgent, more standard work of nonfiction, by no means is that a deal-breaker.

How do people convince themselves -- and seemingly sane people around them -- that God wants them to commit terrible crimes? Should we treat as mentally ill those who say they murdered, raped or kidnapped according to God’s instructions? These critical questions are central to understanding the Smart kidnapping and the murders that are at the heart of Krakauer’s story.

Few real-life characters teeter as far out on the edge as Dan and Ron Lafferty. The brothers come from a Mormon family “admired for their industriousness and probity” (although, Krakauer tells us, their father regularly beat his wife and children and clubbed the family dog to death with a baseball bat in front of them). They grew up to be steady churchgoers and devoted husbands. Then Dan started reading about polygamy.

The Mormon church banned what it called plural marriage in 1890, after the United States government threatened to seize all the church’s assets if it refused to give up the practice. Yet between 30,000 and 100,000 Fundamentalists defiantly participate in plural marriages today, according to Krakauer.

After Dan Lafferty joined forces with a polygamist guru, he declared himself ready for “spiritual wifery” and tried to persuade his five brothers to join him. Ron, the oldest, was the most persuadable. When Ron threatened to marry off his teenage daughters as plural wives, his wife divorced him and took their six children to Florida. He blamed his family’s departure on Brenda, the wife of his youngest brother, Allen, who fought the brothers’ infatuation. Depressed and angry, Ron received what he said was a revelation from God commanding him to “remove” Brenda and her baby. Four months later, he and Dan knocked on Allen’s door, found Brenda and killed her with a butcher’s knife after a struggle. Then Dan walked upstairs to Erica’s crib and slit the child’s throat.

It’s a crime that makes sense only in the upside-down world of faith run amok. Krakauer roots Mormon Fundamentalism in the founding traditions of the church and shows in rich detail the role polygamy played in the lives and theology of the Mormons’ most important figures, Joseph Smith and Brigham Young. “The Lord, it seemed to him,” Krakauer writes of Smith, “must surely have intended man to know the love of more than one wife or He wouldn’t have made the prospect so enticing.”

Krakauer also traces to Smith and Young the Lafferty brothers’ inspiration to kill in the name of faith. It’s a version of history that’s anathema to the official Mormon leadership: Today’s Latter-day Saints hardly agree, as Krakauer writes, that “to comprehend Dan Lafferty” one “must begin with” Joseph Smith, whose “personality still holds extraordinary sway over Mormons and Mormon Fundamentalists alike.” In a statement denouncing Krakauer’s book, church officials correctly point out that he doesn’t include their point of view about the Lafferty murders -- or much of anything else. But their objections, which ignore the preponderance of historic evidence, do little to discount his thesis that there is an enduring link between Mormon-inspired zeal and violent crimes.

To gain insight into the alternative reality of Mormon Fundamentalism, Krakauer takes us to three of its centers: Colorado City, Ariz., a town of 4,000 on the Utah border; Bountiful, a community of 700 in the Canadian bottom-lands; and Colonia LeBaron, home to 3,500 in Chihuahua, Mexico. (In each place, his guide is an insider who has fallen off the Fundamentalist wagon.) Here Krakauer portrays the disturbing webs of family relationships -- with pairs of sisters married to one man and adopted daughters married to their fathers -- that exist in these towns. Giant families live in giant houses, often supported by welfare checks and food stamps. Because polygamy is illegal in every state, Fundamentalist men typically marry only one woman, and the others are considered single mothers. According to Krakauer, the men avoid prosecution for having sex with minors and for plural marriages because in these towns Fundamentalists control the police force and make up the juries.

None of this is good news for the women who decide they’ve had enough. Their stories make for some of the book’s most disturbing chapters: One woman was married at 16 to a 57-year-old man with five other wives and was ordered after his death to marry a violent 54-year-old with four wives. After the woman ran away, she married a third man who sexually molested her 13-year-old daughter. Finally, she burned down her house and fled the town with her five children.

Krakauer wisely lets these stories speak for themselves. Still, his portrayals aren’t entirely satisfying. The reader comes away repulsed rather than moved by these people and their vision of holy living. Krakauer also includes a chapter about Smart. Mostly cobbled from news reports, it doesn’t offer much insight.

In conclusion, Krakauer turns to Dan and Ron Lafferty’s trials for murder. The contrast between the brothers is one of the book’s great strengths. Dan is Krakauer’s most important and intriguing source; interviewed in prison, where he is serving two life sentences, he is cogent and self-assured. “At times when I’ve started to wonder if maybe what I did was a terrible mistake, I’ve looked back and asked myself, ‘What would I have done differently?’ ” he tells Krakauer. “Did I feel God’s hand guiding me on the twenty-fourth of July 1984?’ And then I remember very clearly, ‘Yes, I was guided by the hand of God.’ ”

The sanity of Ron Lafferty, however, was a matter of legal dispute. He wore a sign reading “Exit only” on the seat of his prison jumpsuit to one hearing because of his belief that an evil homosexual spirit was trying to invade his body through his anus. After a jury found him guilty, a federal appeals court worried that he might not have been competent to stand trial because of his belief that he answered only to the laws of God. The court vacated the jury’s verdict -- a decision with potentially unsettling implications. As Krakauer asks, “If Ron Lafferty were deemed mentally ill because he obeyed the voice of his God, isn’t everyone who believes in God and seeks guidance through prayer mentally ill as well?”

If laws are to apply fairly, then the answer has to be no. The state medicated Ron, put him on trial again and found psychiatrists to say that his brand of zealotry wasn’t psychotic. He is now awaiting execution.