SALT LAKE CITY (AP) — A Utah death-row inmate whose double-murder case was featured in the book “Under the Banner of Heaven” and who was nearing execution by firing squad 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.

His case became well known from 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 was convicted in the 1984 slayings of his sister-in-law and her baby daughter, which he carried out with his brother. He claimed he received a revelation from God to kill the two because of the sister-in-law’s resistance to his fundamentalist beliefs in polygamy.

Lafferty was probably only months away from becoming the first American executed by firing squad in nearly a decade after an appeals court rejected his latest his appeal in August.

Lafferty chose the option to be killed by firing squad decades ago when he was sentenced to die. That was before Utah changed its law to use a firing squad for executions only as a backup method if lethal injection drugs were not available.

The last time a firing squad was for an execution in the 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 have argued that he suffered from mental illness and that his death sentence was out of line with the life sentence given to his brother Dan Lafferty.