The heroic "good guy with a gun" who can stop an attack even before law enforcement can arrive has long been a rallying cry by gun rights advocates, arguing that armed citizens can act as a first line of self-defense and public protection.

Police in Bend, Oregon, are still investigating the motive behind the deadly shooting at Silver Rail Elementary School. Four people were killed, including three young children and a teacher. Police identified the shooter as a 16-year-old who previously attended the school. They said he attacked a special education classroom before being killed at the scene. William Brangham reports.

There are many more questions to be answered about the deadly elementary school shooting in Bend, Oregon. To explore some of them, Amna Nawaz spoke with James Densley, professor and chair of criminology and criminal justice at Metro State University. He is best known for co-creating the Violence Prevention Project, a database of mass shooters in the United States.