Q. What's the difference between an active shooter and a mass killer?

A. These and other terms are actually terms used by researchers for a particular research project. That’s why you hear different terms. A current challenge faced by those of us who work in this area of security is developing consistent definitions for these terms.

My team at the FBI researched only active shooter incidents, releasing A Study of Active Shooter Incidents in the United States Between 2000 and 2013. We looked at active shooter situations defined as: "an individual actively engaged in killing or attempting to kill people in a populated area." Other terms are used too such as mass killing, defined by federal statute in 2013 as three or more killed.

In an active shooter situation, it isn't all about the number of people who have been killed or injured. In fact, you could have no one injured. The threat is that there is an individual out in public who wants to kill people. The public can affect the outcome by the way they respond that includes both police and the citizens.

There are other terms better defined in Stop the Killing that are used in the media, such as serial killer, mass shooting, mass casualty. But there is no clearly agreed-upon definition for these, but the research and law enforcement community are closer now than ever before to having universally agreed upon definitions.