Runners, marathon officials and police react as a bomb explodes just a few yards from the finish line of the  Boston Marathon on Monday,  April 15, 2013.  (photo by Ken McGagh)

As smoke fills Boylston Street,  a runner reaches the finish line  and police begin to react moments after a bombing at the finish line of the 2013 Boston Marathon.  (photo by Ken McGagh)

Bill Iffrig, 78, of Lake Stevens, Washington state, is assisted by a race official after collapsing as a Boston Police officer draws her weapon moments after a bombing at the Boston Marathon finish line on Monday, April 15, 2013. (photo by Ken McGagh)

Injured spectators lie in the sidewalk as bystanders begin to react  in front of Marathon Sports on Boylston Street in Boston shortly after a bombing yards away at the Boston Marathon finish line, April 15, 2013. (photo by Ken McGagh)

Smoke hovers over a chaotic scene of injured people and spectators along Boylston Street in Boston, just yards from the Boston Marathon finish line, after a bombing on April 15, 2013. (photo by Ken McGagh)

Good Samaratins  tend to injured spectators on the sidewalk of Boylston Street in Boston after a bombing near the finish line of the Boston Marathon on Monday, April 15, 2013. (photo by Ken McGagh)

Boston marathon competitor  Robert Wheeler kneeling at left, helps a man injured after the bombings at the Boston Marathon finish line, April 15, 2013.  (photo by Ken McGagh)

Boston Firefighter James Plourde carries Victoria McGrath, 20,  an injured spectator,  from the scene of the Boston Marathon bombing on Boylston Street on Monday, April 16, 2013. (photo by Ken McGagh)