A father, who was serving a life term for the horrific act of setting his 2-year-old daughter on fire 26 years prior, passed away from lung cancer in his cell at a well-known and violent prison in Arizona, as revealed by a recent autopsy.

The deceased, Shawn Ryan Grell, 50, was found by prison officials at the Arizona State Prison Complex in Tucson on April 19. At that time, no specific information regarding his passing was released.

A report from the medical examiner obtained by The Post on Wednesday revealed Grell died of natural causes.

According to Dr. Ryan Bruhns, a medical examiner from the Pima County Medical Examiner’s Office, the cause of death was identified as metastatic non-small cell lung cancer. He stated, “The manner of death is natural.”

There were no injuries on his body, according to the report.

Grell’s death generated online speculation over whether it was a result of a revenge killing by another inmate, given the horrifying nature of his crime. 

That speculation came as legislators were calling for sweeping reforms at the Arizona Department of Corrections after three inmates were savagely murdered 15 days before in the same prison complex where Grell died. 

The three prisoners were killed on April 4 by Ricky Wassenaar, a notorious convict who is serving 16 life sentences for murder and his role in one of the longest prison hostage situations in American history, according to prison officials.

Two of Wassenaar’s victims were serving life sentences for sex crimes against a minor, the other was in for raping and murdering a 15-year-old girl. 

“Child molesters: I wanted to kill them all. That’s all I can say,” he told the Arizona Mirror in a recent phone call.

The medical examiner put an end to the online theories.

“This 50-year-old male inmate at the Arizona State Prison Complex – Tucson…was discovered unresponsive in his bed in the Rincon Unit,” the report read. “He was pronounced deceased at the scene by an attending physician.”

Grell’s thin body was delivered to the medical examiner in a white body bag. He weighed just 134 pounds and had bad teeth, according to the report.

Grell was serving a life sentence for murdering his daughter over 25 years ago. He picked up Kristen Salem from a Mesa day care on Dec.  2, 1999, and took her to McDonald’s for lunch, according to court records.

He told the tot they were going to see Christmas lights, Tuscon.com reported, but instead he drove to Target to buy a plastic gas can and gasoline, according to court documents. 

He then drove to 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 woke up and staggered 10 feet while engulfed in flames, before collapsing on the ground.

He later confessed 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.