Medical workers and authorities work on the scene near the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/WCVB-TV/ABC)

Medical workers and authorities work on the scene near the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/WCVB-TV/ABC)