MSNBC Live with Thomas Roberts 09/17/13

Who is DC shooter, and how can we stop next one?

A day after an attack on the Washington Navy Yard, carried out by someone who was once among the military’s own, federal agents are now trying to piece together why. Investigators say that the 34-year-old gunman, Aaron Alexis, was troubled and had run-ins with the law, but some friends say they still never saw this coming. NBC's Pete Williams reports. ... more Duration: {{video.duration.momentjs}}