A Utah death-row inmate who killed his brother's wife and her toddler in 1984 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 in a statement.

The state attorney general's office had predicted he would have been executed in 2020, becoming the first American executed by firing squad in nearly a decade, after a court rejected his latest his appeal in August. CBS affiliate KUTV reports Lafferty's legal team was in the process of appealing to the full 10th U.S. Circuit Court of Appeals when he died.

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."

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Lafferty carried out the killings with his brother Dan, who received a life sentence. Ron Lafferty claimed he had received a revelation from God to kill Brenda Lafferty and her 15-month-old daughter, Erica, because of the sister-in-law's resistance to his fundamentalist belief in polygamy.

Utah death row inmate Ron Lafferty is seen October 24, 2011, in a photo released by the state Department of Corrections. Utah Dept. of Corrections via AP

The killings happened in American Fork, Utah, about 30 miles south of Salt Lake City. Brenda Lafferty was beaten and strangled with a vacuum cleaner cord.

She and Erica died after their throats were slashed. Ron Lafferty was found guilty by a jury in 1985 and sentenced to death.

A federal appeals court in 1991 overturned his conviction because a trial judge used an incorrect legal standard to find Lafferty competent to stand trial. He was retried in 1996, found guilty by a jury and again sentenced to death.

Ron Lafferty chose to be killed by firing squad decades ago instead of receiving a 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.

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. 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 eldest 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 from the faith in 1983.

They joined a renegade polygamist cult called the "School of Prophets" and Ron Lafferty left behind his clean cut and well-groomed look for long hair and an unkept beard. Ron Lafferty's 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 July 24, 1984. According to a friend of the brothers who was waiting for them in a car outside the house, she screamed: "Don't hurt my baby! Please don't hurt my baby!"