November 17, 2014 12:00 PM

There were hints of trouble: Benjamin Strack missed work as a bricklayer for about a week. Extra food was laid out for the family pets. And in the days before the Stracks—Benjamin, 37; wife Kristi, 36; and their children Benson, 14; Emery, 12; and Zion, 11—were found dead in their Springville, Utah, duplex, Benson wrote a goodbye letter to a friend. “He was aware something was going to happen,” his uncle Isaac Strack tells PEOPLE, “that he might be found dead someday.”

That someday was Sept. 27, when Benson’s half brother, Janson, 18, came home to cars in the driveway but the house quiet, with the master bedroom locked. He called his grandmother Valerie Sudweeks, who helped force the door open on a horrific scene—one that left neighbors at first fearing a mass murderer, while the grieving family struggled with suspicions that Benjamin and Kristi had mental illness that led to tragedy.

As described in a police affidavit, Benjamin and Kristi, with a red liquid dripping from her mouth, were dead in the bed while the children’s bodies “were lying around the bed, covered in bedding up to their necks.” Beside each child was a cup. Officers first suspected a gas leak, but Sudweeks knew better. “Valerie said she couldn’t believe ‘she’ would do this to the kids,” the affidavit said. Empty sleeping-pill boxes were among the evidence carted away.

In the weeks since, authorities have declined to comment pending autopsy and toxicology results due at the end of November. But when relatives cleaned the house, Isaac says, they found “preparations as though [the family] were going on a long trip” and other signs the deaths were “intentional.” He and Janson’s uncle Bob McGee, along with a close family member who prefers anonymity, consulted the National Alliance on Mental Illness (NAMI.org) and spoke with PEOPLE in hopes of sparing another family similar devastation. “Kristi and Ben were victims of their minds,” the relatives said.

The couple had a history of substance abuse. To another relative’s mention of delusions and paranoia, Isaac resisted getting into specific symptoms, saying, “They had some warning signs that we, not being doctors, weren’t able to recognize.” Says McGee: “The only way they’d do this was that they believed they were protecting their children.”

Now the family is rallying around Janson, who is seeing a therapist and trying to stay focused on plans for college or trade school. They all take small solace in believing that only Benson, with his goodbye letter still in his notebook, sensed what his parents were about to do, says Isaac. “We hope that Emery and Zion weren’t scared, that they died peacefully.”

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