(CNN) Columbine survivor and addiction speaker Austin Eubanks died of "acute heroin toxicity," the coroner's office has confirmed.

Eubanks, who was found dead last month during a welfare check after he didn't answer his phone, was 37.

Eubanks' death has been "ruled an accidental heroin overdose," Routt County, Colorado, Coroner Robert Ryg, told CNN Thursday.

"I'm very sad about it myself," Ryg said. "We were hoping for something else, heart attack or something."

Eubanks, a survivor of the 1999 Columbine massacre, struggled with opioid addiction after the shooting and later became a public speaker discussing the issues of substance abuse in the country, according to his website

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