Quite often it is only in hindsight that the friends and family of a mass-shooting perpetrator realize that the bloodbath was actually the climax of a series of foretelling clues from an angry, violent, paranoid, withdrawn person, who happens to have a  newfound fixation with guns. Such devastating epiphanies beg the question: Can we predict tomorrow's mass shooter?

Dr. Peter Langman, Ph.D, one of the foremost experts in the psychology of school shooting perpetrators, talks to A&E True Crime about who commits these heinous crimes, why they do it and what we can do to prevent the next shooting.

Kristina Anderson, survivor of the Virginia Tech mass shooting, tells her story of that tragic day and how to prepare for an active-shooter scenario.