As Boston Marathon runners crossed the finish line on April 15, 2013, three people were killed and at least 133 people were injured, when bombs exploded almost simultaneously. For the next several days, authorities worked to identify the cause of the explosions and the people responsible for the attacks. For full coverage, check out Boston Marathon Explosion
, Three Dead, Including Child, in Boston Explosions
, and One Boston Marathon Bombing Suspect Dead, 'Massive Manhunt' for Second."