In the 20 years since Columbine, researchers have dedicated themselves to studying potential shooters and teaching others how to spot them. They believe that assessing individual threats - based on psychology and behavior - can prevent attacks.

Locks, Lights, Out Of Sight

On April 20, 1999 a school shooting in Littleton, Colo., shook the country. Now, 20 years later, Reporters Nate Minor and Andrea Dukakis look at how one shooting changed America and what we've learned since.