What Are the Real Warning Signs of a Mass Shooting?

The freshman who walked into the high school cafeteria in Marysville, Wash., in 2014 with his father’s .40-caliber Beretta did not fit anyone’s profile of a mass murderer. He was a crack athlete. He embraced his Native American traditions, wearing a headdress at tribal events and offering freshly killed deer to his grandmother. He was popular, so much so that he had just been elected homecoming prince.
He had no history of mental illness — just what several classmates described as an uncharacteristically bad mood that week. It was only...