An Arizona dad who deceived his 2-year-old daughter by promising to take her to see Christmas lights but instead took her into the desert and set her on fire has passed away in prison 26 years after the heinous crime.

The body of convicted murderer Shawn Ryan Grell, 50, was discovered by officials in the Arizona Prison Complex Tucson on April 19, but they did not disclose any details about his death.

“All inmate deaths are investigated in consultation with the county medical examiner’s office,” the Arizona Department of Corrections, Rehabilitation and Reentry said in a statement to The Post Wednesday.

The Prima County Medical Examiner’s Office could not be reached for comment. 

Grell case was described by Arizona’s highest court as “horrific,” according to azfamily.com.

Grell picked up his young daughter, Kristen Salem, from a Mesa day care on Dec.  2, 1999, and took her to McDonald’s, the outlet reported.

He said they were going to see Christmas lights, Tuscon.com reported in 2009, but instead drove to Target to buy a plastic gas can and gasoline, according to court documents. 

He drove to a deserted area in the desert outside Mesa, laid his sleeping daughter on the ground, doused her in gasoline and lit her on fire, the records said.

Little Kristen awoke, stumbled 10 feet while engulfed in flames, before collapsing on the ground.

After killing his daughter, Grell went to buy beer at a nearby convenience store. He told the clerk that he had seen some teenagers setting a dog on fire in a vacant lot. 

He drove around for hours before calling the cops and turning himself in. He would later confess to the killing at a press conference early the next morning. 

“I took the gasoline and I poured it on her,” he told investigators during a videotaped interview. “I took the match and threw it on her,” he added with no trace of emotion in his voice. 

The court ruled that the senseless killing was committed in an “especially heinous, cruel, and depraved manner” and Grell was charged with first-degree murder and sentenced to death. 

But that was overturned via a unanimous ruling by the Arizona Supreme Court in 2013, which found his death sentence was “cruel and unusual punishment.”

Grell was one of 144 death sentences around the country affected by a Supreme Court decision in 2002 that ruled it was a violation of the Eighth Amendment to administer the death penalty to people with “mental deficiencies.”

“Because of their disabilities in areas of reasoning, judgment, and control of their impulses, however, they do not act with the level of moral culpability that characterizes the most serious adult criminal conduct,” the Supreme Court ruled in Atkins v. Virginia.   

Grell’s lawyers successfully argued, citing the killer’s low IQ scores and other medical tests, that their client met the criteria laid out by the Supreme Court in their 6-3 decision.