The bodies of the parents were found alongside their three children in their Utah home on Sept. 27

Relatives of a family of five found dead last month in the upstairs bedroom of their Springville, Utah, home, tell PEOPLE exclusively that they believe Benjamin and Kristi Strack took their own lives and the lives of their three children because the couple suffered from long-term mental illness and substance-abuse disorders.

The Stracks were found dead by an older son, Janson McGee, with cups of red liquid next to their bodies, on Sept. 27. The children – Benson, 14, Emery, 12, and Zion, 11 – were found lying on and around the bed, covered with blankets up to their necks, with empty bottles of liquid methadone and boxes of cold and flu medication nearby.

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Investigators have determined the cause of death to be accidental or intentional poisoning (autopsy results won’t be released until the end of November), but Benjamin Strack’s brother says the deaths were no accident.

Isaac Strack, 39, says family members cleaning out the Stracks’ home a few days after the grim discovery found a notebook with a letter inside, written by Benson Strack to one of his best friends. “It was pretty clear that it was a goodbye letter,” says Strack. “It indicated that Benson was aware that something was going to happen. Benson at least had some idea that he might be found dead someday.”

He and other distraught family members say that looking back, they now realize that Benjamin, 37, and Kristi, 36, were showing signs of mental illness, especially in the weeks before their deaths. “They had isolated themselves from neighbors and some of the family,” he says, “and it had been awhile since Ben had been to work.”

Isaac and another relative, Bob McGee, Janson’s uncle, say that Benjamin and Kristi Strack were also exhibiting “irrational fear,” afraid that something or someone was going to harm them.

“Their mental illness combined with substance-abuse disorder is what led to this tragedy,” says Isaac. “Some of the changed behaviors before their deaths seemed out of place and didn’t make sense at the time. But looking back now, they make sense. And now we want to speak out to prevent other people from having to endure what we are enduring.”

Extended family members believe that the couple were “victims of their minds,” says McGee, with their mental illness caused and made worse by drug use. “This one action does not define the Ben and Kristi that we remember,” he says. “They were caught in a vicious cycle that many people face in this world. There needs to be a greater sense of dialogue and urgency in our country regarding mental illness. It is simply a disease of the mind, and like any disease, we should work to cure it and prevent it.”

Lt. David Caron of the Springville Police Department told PEOPLE that while he appreciates the family members’ concerns for people suffering from mental illness, it would be inappropriate to make any comment concerning the case.

“Our contacts with the family were fairly limited in scope,” he said. “Therefore I would defer to the family members who both knew and loved Ben and Kristi, and who are grappling with the sorrow caused by this tragic event.”

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