A man whose granddaughter and son were killed in a brutal attack in Montana over the weekend says he is urging surviving family members not to “harbor anger” against the alleged killer. Derick Amos Madden, 37, is accused of using a shotgun to kill 18-month-old McKenzie Siau and her father, 39-year-old David Siau, as the family walked on a roadside in East Glacier on Sunday night. Siau’s wife, Christy, was left in critical condition after the ambush, which appeared to have been targeted at Madden’s ex-girlfriend, David Siau’s sister. The Daily Beast reported Madden was an Afghanistan war veteran who returned home plagued by his experience, and had mental-health complications that “stemmed from that,” according to his sister.

In a video message released by the FBI late Wednesday on behalf of the Siau family, John Siau revealed how they were coping after the tragedy. “As I have talked to each of my children, we have a very large family, I said to them, ‘Grieve, be sad, but don’t harbor anger about what has happened,’” he said. “Because what we have seen is actually the result of somebody who has harbored anger in his own life and allowed it to fester and allowed it to grow and develop into something very terrible and unspeakable.”