After dark on Sept. 27 last year, a Springville, Utah, teen returned home with his girlfriend. On a normal evening, he would have seen his parents and three siblings. But that night, he couldn’t find anyone. He couldn’t hear anything. And when he went to his parents’ bedroom, he couldn’t open the door. He called his grandmother, who forced her way inside.

They found five dead bodies spread across the room.

“Oh my god,” family friend Maureen Ledbetter told a 911 dispatcher, as the grandmother screamed in the background. “The whole family killed themselves.”

Since then, what exactly happened to Benjamin and Kristi Strack and their three youngest children seemed a mystery. And, in many ways, it still is.

But on Tuesday, police revealed what they found when they responded to the 911 call from one of the worst murder-suicide scenes in Utah’s history.

When police got to the home in the small town about 50 miles from Salt Lake City, they found Benjamin and Kristi Strack and three of their four children dead inside the master bedroom. They found empty containers of cold and flu medication, allergy medication, sleeping medication, pain reliever and cherry-flavored liquid methadone. And they found a plastic sand pail filled with a lethal yellowish-orange concoction, a mixture of the drugs.

It’s still unclear how the children consumed the cocktail.

The Utah State medical examiner said the children, ranging in age from 11 to 14, had toxic amounts of diphenhydramine, which is an antihistamine, and methadone in their bodies. Kristi Strack had the same drugs, plus dextrophan and doxylamine. And Benjamin Strack had toxic levels of heroin in his system, the Salt Lake Tribune reported.

The parents’ deaths were listed as suicides. The two youngest children’s deaths were ruled homicides because they were too young to consent to die. The death of their 14-year-old son was labeled “undetermined.”

“Because of his age, it is difficult to determine if he was capable to make a decision to commit suicide or to consent to join with his parents in committing suicide,” Springville police said in a news release.

Much less certain, and perhaps more perplexing, is why they wanted to die. But relatives believe mental health issues played a role in their decision.

Friends and family told police that the parents were worried about the “evil in the world” and wanted to escape a “pending apocalypse.” But most assumed they just wanted to move somewhere “off the grid.” In the home, police found a to-do list with instructions to “feed the pets” and “find someone to watch over the house.” They also found a note from the Stracks’ 14-year-old “bequeathing his personal possessions to his friend,” Springville Police Chief J. Scott Finlayson said at a news conference.

Investigators believe the cocktail put the children to sleep before it killed them. Benson, 14, and Emery, 12, were discovered on mattresses on the floor in their parents’ bedroom. Benjamin and Kristi Strack and their 11-year-old son, Zion, were on the parents’ bed, police said. Investigators believe Benjamin Strack was the last to die, after climbing between his wife and youngest son on the bed, the Salt Lake Tribune reported.

There were no suicide notes — only the letter that 14-year-old Benson wrote to a friend two days before the family died.

“It was a goodbye letter to a friend. And it basically mentioned that he would no longer exist on this Earth,” Benjamin Strack’s brother Jake told the Deseret News. “Just saying goodbye, and he had an inkling he would be found dead.”

Authorities believe their only surviving child, 19-year-old Janson McGee, who found his family dead, was not included in the murder-suicide because he was grown and engaged to be married, the newspaper reported.

Investigators also found letters written by Kristi Strack to one of the state’s most infamous convicted killers, Dan Lafferty, who was convicted in the 1984 fatal stabbing of his sister-in-law and her 1-year-old daughter. According to trial testimony, he killed the victims at the order of his brother, Ron Lafferty, who claimed to have had a revelation from God. The story became a book called “Under the Banner of Heaven.”

Police said Kristi Strack became friends with Dan Lafferty, and she and her husband even visited him in prison.

“It was almost like he talked to her like one of his children,” Lt. David Caron said, according to the Associated Press.

Police said Lafferty had asked the Stracks to handle his remains when he died, though they don’t believe the couple had seen him since 2008. Police went to question Lafferty after the Stracks’ suicide pact. He said he no idea about it.

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

According to news reports, Kristi Strack had a history with heroin. She was receiving methadone treatments for heroin use, the Deseret News reported.

Alex Short, who owns a bricklaying company where Benjamin Strack worked, told the AP that, over the past several years, Strack had skipped work for long stretches of time. He told Short he was helping his with wife with things. Before the murder-suicide, Strack had been absent for more than a week, though Short said he thought the couple’s troubles were in the past.

“He was kind of at the tail end of all those problems,” he said.

Relatives said there was also frequent talk of leaving this world.

“There seemed to be a concern about a pending apocalypse that the parents bought into,” Finlayson told reporters. “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.”

“There are some questions we can’t answer and may never be able to answer,” Finlayson added.