ST. PAUL, Minn. (AP) — Two Twin Cities researchers are building a database of mass shooters with the goal of better understanding why mass shootings happen and identifying ways to prevent them.

Minnesota Public Radio reports that Jillian Peterson, a Hamline University assistant professor of criminology and criminal justice, and James Densley, an associate professor of criminal justice at Metropolitan State University, are working on the project.

The researchers are focusing on shootings that occurred in a public place and had four or more victims. The shootings are also not family or gang-related. They code shootings based on 50 different variables.

Peterson says they research shooters' past trauma, their family, mental illness, relationships with other people and social media profiles.