Community members grieve at the site of a makeshift memorial in front of Silver Rail Elementary School in Bend, Oregon, on October 16. - Mathieu Lewis-Rolland/Getty Images

On a sunny Oregon fall morning, parents dressed their children up and sent them to Silver Rail Elementary School for picture day. It’s the kind of picture parents return to years later, marveling at how small their children once were and how their smiles looked before their faces changed.

But for some families, snapshots in their minds from that morning will carry a devastating, final memory of a child who never came home.

Just hours after Steven...