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 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 Jon 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 tucked into covers in and around their parents’ bed.

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.