Medical workers aid injured people at the 2013 Boston Marathon following an explosion in Boston on Monday, April 15, 2013. Two explosions shattered the euphoria at the marathon's finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Globe, David L Ryan)

Photo by: David L. Ryan

Medical workers aid injured people at the 2013 Boston Marathon following an explosion in Boston on Monday, April 15, 2013. Two explosions shattered the euphoria at the marathon's finish line on Monday, sending authorities out on the course to carry off the injured while the stragglers were rerouted away from the smoking site of the blasts. (AP Photo/The Boston Globe, David L Ryan)