(CNN) The mysterious deaths of five Utah family members in their Springville home baffled authorities at first.

Benjamin and Kristi Strack were found dead on their bed on September 27. On the floor near them in the locked master bedroom lay the bodies of three of their children, from 11 to 14 years old.

There were no signs of violence, but beside each body, there were cups with liquid inside.

On Tuesday, police in Springville revealed the conclusion of their investigation: The parents had committed suicide by taking toxic levels of drugs and had given fatal levels of drugs -- including the heroin substitute methadone -- to their younger two children.

Their 14-year-old son, Benson, also had taken toxic levels of drugs, but the manner of death was ruled "could not be determined" because investigators were unsure whether he was capable of deciding to join his parents in committing suicide.

But why would a family do this?

According to a statement released by J. Scott Finlayson, chief of police for Springville, the parents had bought into "a concern about a pending apocalypse."

From interviews with friends and relatives of the Strack family, the statement said, "it became fairly apparent that the topic of 'leaving' this world was a fairly common theme."

It added, "While some friends thought 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."

Tragically, the parents chose the former option, police believe, and took their children with them.

No suicide notes

They didn't leave behind many clues to their thinking. Investigators found a notebook with the kind of handwritten lists parents write before going on vacation -- like feeding the pets and asking someone to watch after the house -- but no suicide notes were uncovered.

However, one note was found apparently written by Benson to a friend, "which indicated that Benson was aware that he may die, and was bequeathing his personal possessions to his friend," the police statement said.

"This was the only letter or note found that gave any indication that family members knew what may transpire in the home."

It was this note that led investigators to conclude that Benson might have known that he was going to die, having been heavily influenced by his parents' apocalyptic beliefs, and could have agreed to the plan.

"The other two children were obviously too young, at ages eleven and twelve, to consent to any sort of agreement to commit suicide," police said of his sister, Emery Strack, and brother, Zion Strack.

This, and the fact that their parents must have provided the drugs, led police to determine that their deaths were homicide.

Autopsy reports

The medical examiner's report said the drugs that caused the children's deaths were methadone and diphenhydramine, a common over-the-counter antihistamine used in such medications as Benylin and Nytol.

The autopsy for Kristi Strack, 36, revealed the cause of death to be "drug toxicity," with methadone, dextrorphan, diphenhydramine and doxylamine found in her system.

She was receiving methadone treatment, police said, so she would have known just how toxic this combination of drugs would be. This contributed to their determination of suicide.

Her husband, Benjamin Strack, 37, was found to have a toxic level of heroin in his body.

Investigators say they think he was the last to die, his family around him.

"His arm and leg were draped over Kristi. He was on top of the bed covers; while the other family members were under bed covers. This would indicate that he was likely the last in the family to succumb to the toxic levels of drugs in his system," the statement said.

This was the awful scene that confronted another son of Kristi Strack when he and a girlfriend, who'd noticed an eerie quiet in the house, got inside the locked bedroom.

In an audio recording of the 911 call made from the house, also released by police, an emotional-sounding young woman can be heard saying that "the whole family is dead" and that they've "killed themselves."

Finlayson expressed his condolences to Strack family relatives, saying the unexpected nature of the deaths must have intensified their shock and grief.

"In a case such as this one, where this family has lost not one, but five family members, the deep emotional pain must be incredibly difficult. Our hearts and prayers go out to the family," his statement said.