Scenes from the Boston Marathon Tragedy

Normally the finish line at the Boston Marathon is a scene of relief, celebration, pride and joy. But after just half the marathoners had crossed, two explosions 12 seconds apart turned that scene into chaos and tragedy. At least three people died and a hundred or more were injured, according to police reports. The finish line is now a crime scene, locked down, as police search through dropped backpacks, trashcans, videos and photos for clues to who may have left bombs in the last few blocks of the marathon.

Anchor Marco Werman talks to Homeland security expert and Boston Globe columnist Juliette Kayyem to get more details.