SPRINGVILLE, Utah » A Utah couple and their three children who were found dead in their home last fall overdosed on drugs after the parents told friends they were concerned about a pending apocalypse, authorities said Tuesday.

Police also found old letters from the mother to a Utah man in prison for killing family members in the name of God, murders that were 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.

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 left a goodbye letter bequeathing 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 concerned with 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 might 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 for a double murder he committed 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 it appeared the Stracks were close with Dan Lafferty at one point but hadn’t talked to him since 2008. Police Corp. Greg Turnbow said investigators do not believe the couple’s beliefs came from Dan Lafferty.

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’ bodies were found by their older son and the children’s grandmother Sept. 27, according to search warrants. Police found cups with liquid inside next to each of the bodies.

The older son was Kristi Strack’s child from a previous marriage.

In a recording of the 911 call released Tuesday, family friend Maureen Ledbetter tells the dispatcher that the whole family killed themselves as grandmother Valerie Sudweeks is heard screaming 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. But 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.