SPRINGVILLE, Utah (AP) — A Utah couple and their three children who were found dead in their home last fall overdosed on drugs after the parents told friends and family they were worried about the apocalypse, authorities said Tuesday.

Police also found old letters written by the mother to a Utah inmate serving time for killing family members in the name of God, slayings chronicled in the 2003 John Krakauer book “Under the Banner of Heaven.”

Benjamin and Kristi Strack and three of their four children — ages 11, 12 and 14 — were found dead in September in a locked bedroom of their Springville home. All five were in a bed, with the kids tucked into the covers around their parents.

At a news conference Tuesday, Springville Police Chief J. Scott Finlayson said investigators have concluded their probe and determined the family members died from drug toxicity from either methadone, heroin or a combination of drugs, including those found in cold medicine.

Authorities determined the parents committed suicide. The younger two children’s deaths were ruled homicides, although Finlayson said there were no signs of a struggle.

The manner of death for the 14-year-old, Benson Strack, was undetermined.

Police said Benson wrote a goodbye letter, leaving some of his belongings to his best friend. The only other recent writing the family left behind was a notebook containing handwritten to-do lists about feeding the pets and other chores.

Finlayson said interviews with people who knew the Stracks indicated the parents were worried about evil in the world and wanted to escape from “impending doom.”

“There seemed to be a concern about a pending apocalypse that the parents bought into,” Finlayson said. “While some friends though that suicide may have been, or could have been, included in their plans, others believed they were going to move somewhere and live off the grid.”

During their investigation, police found years-old letters between Kristi Strack and Dan Lafferty, who is serving a life sentence after being convicted of committing a double-murder with his brother Ron.

“Under the Banner of Heaven” is about members of an offshoot of The Church of Jesus Christ of Latter-day Saints, told through the true story of the Laffertys’ crimes. The two killed their brother’s wife and 15-month old daughter.

Ron Lafferty was condemned to death for the July 1984 slayings of his sister-in-law, Brenda Lafferty, and her baby in American Fork. He claimed to have had a religious revelation sanctioning the slayings because of the victim’s resistance to his beliefs in polygamy.

Investigators said Kristi Strack was deeply interested in the case and struck up a friendship with Dan Lafferty.

“It was almost like he talked to her like one of his children,” Lt. David Caron said. She and her husband both became close to him, and he had directed his remains to go to them after his death. They hadn’t talked to him since 2008 and investigators do not believe the couple’s beliefs came from Dan Lafferty, police Cpl. Greg Turnbow said.

“He felt really sad they had committed suicide,” Caron said.

Benjamin Strack’s brother Jacob said the final report from investigators wasn’t surprising to relatives still mourning the family’s deaths.

Looking back, the connection to Dan Lafferty was a troubling sign about the couple’s mental state, he said.

The Stracks’ older son and the children’s grandmother found the bodies Sept. 27, according to search warrants. The older son was Kristi Strack’s child from a previous marriage.

Police found cups with liquid inside next to each of the bodies and a child’s sand bucket behind a door with traces of the same combination of methadone and cold medication that was found in their systems.

Police believe that Benjamin Strack died last because he was the only member of the family who wasn’t underneath the bed covers.

In a recording of the 911 call released Tuesday, family friend Maureen Ledbetter tells the dispatcher about the deaths as grandmother Valerie Sudweeks screams in the background.

The methadone used in the deaths was prescribed to Kristi Strack, police said.

Court records show Benjamin and Kristi Strack had a history of legal and financial problems and had gone through court-ordered drug treatment several years ago. Investigators said they weren’t aware of any contact with state child services workers.

Benjamin Strack’s boss has said it appeared those troubles were behind them.

Springville is a city of about 30,000 near Provo, about 45 miles south of Salt Lake City.