A potent predictor Peterson’s database has revealed is suicide: the shooters are almost always suicidal. Her latest analysis shows that 80 percent of the perpetrators were suicidal before the shooting, which she says it is key to understanding their actions.

“In every case, this was intended to be their final act,” said Peterson, a professor of criminal justice at Hamline University in Minnesota. “They are murders, but they are murder-suicides. Meaning, the perpetrator doesn’t intend to make it through the shooting — they are either killed by police, they shoot themselves, or they spend the rest of their lives behind bars.”

One critical pathway to stop future massacres could be to focus on suicide prevention, Peterson believes.

“We know more about the field of suicide prevention in terms of what works, and those same strategies might be effective here,” she said.