Each time a high-profile mass shooting happens in America, a grieving and incredulous nation scrambles for answers. Who was this criminal and how could he (usually) have committed such a horrendous and inhumane act? A few details emerge about the individual’s troubled life and then everyone moves on.

Three years ago, Jillian Peterson, an associate professor of criminology at Hamline University, and James Densley, a professor of criminal justice at Metro State University, decided to take a different approach. In their view, the failure to gain a more meaningful and evidence-based understanding of why mass shooters do what they do seemed a lost opportunity to stop the next one from happening. Funded by the National Institute of Justice, the research arm of the Department of Justice, their research constructed a database of every mass shooter since 1966 who shot and killed four or more people in a public place, and every shooting incident at schools, workplaces and places of worship since 1999.