According to the Department of Homeland Security, an active shooter is “an individual actively engaged in killing or attempting to kill people in a confined and populated area. In most cases, active shooters use firearms and there is no pattern or method to their selection of victims.” These deadly events happen about 18 times per year in the Unites States. Active shootings are distinct from mass shootings, which are defined as incidents where four or more people are shot.

After the shooting at Columbine High School, The Advanced Law Enforcement Rapid Response Training (ALERRT) Center was created to instruct police officers in active-shooter response. Contributing producer Kelly Loudenberg went inside ALERRT courses to uncover how the nation is attempting to respond to the recent rise in mass killings.

With Great Power is an Atlantic project on police, crime, and guns in America, supported by a grant from the Joyce Foundation.