SALT LAKE CITY (AP) — A Utah death-row inmate who killed his brother's wife and her toddler because of his strong polygamist beliefs in a case made famous by the book "Under the Banner of Heaven" has died of natural causes, prison officials said Monday.

Ron Lafferty, 78, died at the state prison in the Salt Lake City suburb of Draper, Utah Department of Corrections spokeswoman Kaitlin Felsted said.

The state attorney general's office had expected him to be executed next year and become the first American put to death by firing squad in nearly a decade.

Lafferty's case became well-known after it was featured in Jon Krakauer's 2003 book about radical offshoots of The Church of Jesus Christ of Latter-day Saints. Krakauer also wrote "Into Thin Air" and "Into the Wild."

Lafferty claimed he had received a revelation from God to kill the mother and child in 1984 because of the sister-in-law's resistance to his fundamentalist belief in polygamy. Lafferty carried out the killlngs with his brother, Dan Lafferty, who received a life sentence.

Brenda Lafferty was beaten and strangled with a vacuum cleaner cord. She and her 15-month-old daughter, Erica, died after their throats were slashed.

The killings occurred in American Fork, Utah, south of Salt Lake City. Ron Lafferty was convicted by a jury in 1985 and sentenced to death, and again in 1996 after a federal court had overturned the first conviction.

He chose decades ago to be killed by firing squad instead of lethal injection, a choice given at the time to death row inmates. Utah later changed its law, allowing firing squad executions only as backups if lethal injection drugs were not available.

Krakauer said he's grateful Brenda Lafferty's relatives won't be exposed to a firing squad execution. Krakauer got to know her sisters while researching his book and said ongoing appeals in the case have taken a toll on the family.

"That would have been a circus," Krakauer said of a firing squad. "That would have been horrible for everybody."

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Utah Attorney General Sean Reyes called it "cruel and tragic" that the case lingered so long and said Monday he hopes Lafferty's death brings "ultimate justice" and some closure for the family of the victims.

Krakauer said news of Lafferty's death made him sit down while he was in the mountains and flooded him with disturbing memories about the brothers he called "horrible sociopaths" who "ruined so many lives."

"They used their religious beliefs as a license to do what they wanted," Krakauer said. "It was a horribly upsetting book to write."

Ron Lafferty's lawyers had argued that he suffered from mental illness and that his death sentence was out of line with the life sentence given to his brother.

The mainstream, Utah-based Church of Jesus Christ of Latter-day Saints disavowed polygamy more than a century ago, but some church offshoots still practice it and consider themselves "fundamentalist Mormons."

Ron Lafferty was the oldest of eight children and raised in a Utah family that belonged to the mainstream church, according to court documents.

For years, he followed the script for successful young church members. He served a church mission in Florida and then returned to Utah to get married, have children and get a job. He also served on voluntary councils for his congregation and the community.

But in 1982, Lafferty began spending more time with his brother Dan, who believed the mainstream church should have never abandoned polygamy. Ron Lafferty eventually adopted his brother's thinking and the two were excommunicated in 1983.

They joined a renegade polygamist cult called the "School of Prophets." Ron Lafferty left behind his clean cut and well-groomed look and drew long hair and an unkept beard. His wife divorced him and moved with their children to Florida.

By 1984, the brothers had become enraged by the strong opposition to polygamy by Brenda Lafferty, the wife of their brother Allen. She had agreed with the decision by Ron Lafferty's wife to leave him.

Ron Lafferty believed he had received a revelation from God to kill Brenda Lafferty and her daughter, and the brothers went to her apartment on July 24, 1984.

According to Charles "Chip" Carnes, a friend who waited for them in a car, she screamed: "Don't hurt my baby! Please don't hurt my baby!" The killers left the back of the apartment covered in blood, Carnes said.

Allen Lafferty later discovered his wife lying in a pool of blood in the kitchen and their daughter propped against her crib.

The last time a firing squad was used for an execution in the U.S. was in 2010, when Ronnie Lee Gardner was executed in Utah for the 1984 murder of an attorney during a failed courthouse escape.