A Utah death row inmate who wanted to be executed by firing squad following his conviction for murdering his sister-in-law and her young daughter over his strong polygamist beliefs has died of natural causes in prison, corrections officials said Monday.

Ron Lafferty died at a state prison in Draper, a suburb of Salt Lake City, a Utah Department of Corrections spokeswoman said. The 78-year-old was sentenced to death in 1985 for the killing of his brother's wife and her young child because of her resistance to polygamy.

The murders were the basis for Jon Krakauer's 2003 book "Under the Banner of Heaven" A Story of Violent Faith."

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This August photo, released by the Utah Department of Corrections, shows Ron Lafferty. (Utah Department of Corrections via AP, File)

Lafferty claimed he killed the two because of a revelation from God. His brother, Dan, 71, received a life sentence for his involvement in the slayings.

On July 24, 1984, Brenda Lafferty was beaten and strangled with a vacuum cleaner cord in her apartment in Fork. She and her 15-month-old daughter, Erica, had their throats slashed.

Allen Lafferty discovered his wife in a pool of blood and their daughter near her crib.

Ron and Dan Lafferty believed their sister-in-law's refusal to join the School of Prophets interfered with a "divine," plan that required the participation of all six Lafferty brothers, The Salt Lake Tribune reported.

A jury found Ron Lafferty guilty and sentenced him to death. He was retried and again sentenced to death in 1996 after a federal appeals court overturned the conviction because a judge used an incorrect standard to determine he was fit to stand trial.

In 2005, Ron Lafferty, right, confers with his attorney. (AP Photo/George Frey, File)

At the time, Utah's death row inmates were allowed to choose their preferred method of execution. Lafferty chose a firing squad. The state later changed the law to use firing squads only as a backup to lethal injections.

Ronnie Lee Gardner was the last inmate to die in the U.S. by firing squad, back in 2010. Utah authorities executed him after he was convicted of killing an attorney during a failed courthouse escape.

The state attorney general predicted Lafferty would have been executed in 2020.