Explosions at Boston Marathon

An unidentified Boston Marathon runner leaves the course crying near Copley Square following an explosion in Boston Monday, April 15, 2013. (AP Photo/Winslow Townson)

Medical workers wheel the injured across the finish line during the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon 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/Charles Krupa)

In this photo provided by The Daily Free Press and Kenshin Okubo, people react to an explosion at the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon 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 Daily Free Press, Kenshin Okubo) MANDATORY CREDIT

A Boston police officer clears Boylston Street following an explosion at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the 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/Charles Krupa)

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. (AP Photo/Charles Krupa)

In this image from video provided by WBZ TV, spectators and runners run from what was described as twin explosions that shook the finish line of the Boston Marathon, Monday, April 15, 2013, in Boston. Two explosions shattered the euphoria of the Boston Marathon 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/WBZTV) MANDATORY CREDIT

A flag flies over the finish line as medical workers aid injured people following an explosion at the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria at the finish line, 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/Charles Krupa)

A Boston police officer wheels in injured boy down Boylston Street as medical workers carry an injured runner following an explosion during the 2013 Boston Marathon in Boston, 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/Charles Krupa)

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon 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/Charles Krupa)

Women react as they walk from the area where there was an explosion after the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Josh Reynolds)

A firefighter tends to an injured man following an explosion near the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Michael Dwyer)

Boston police on bicycles patrol on Commonwealth Avenue following the explosions at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded at the Boston Marathon finish line Monday killing two people and injuring dozens. (AP Photo/Michael Dwyer)

One of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon is investigated by two people in protective suits in the wake of two blasts in Boston Monday, April 15, 2013. (AP Photo/Elise Amendola)

President Barack Obama leaves the podium after speaking in the James Brady Press Briefing Room at the White House in Washington, Monday, April 15, 2013, following the explosions at the Boston Marathon. (AP Photo/Charles Dharapak)

Boston police clear an area following an explosion near the finish line of the 2013 Boston Marathon in Boston, Monday, April 15, 2013. Two explosions shattered the euphoria of the Boston Marathon 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/Charles Krupa)

Runners who were diverted from the race course walk on the Commonwealth Mall two blocks from the site of an explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded near the finish line of the Boston Marathon on Monday, killing at least two people and injuring at least 23 others. (AP Photo/Michael Dwyer)

Justine Franco of Montpelier, Vt., holds up a sign near Copley Square in Boston looking for her missing friend, April, who was running in her first Boston Marathon Monday, April 15, 2013. Two bombs exploded near the finish line of the marathon on Monday, killing at least two people and injuring at least 23 others. (AP Photo/Winslow Townson)

Emergency responders gather around the finish line of the Boston Marathon following an explosion in Boston Monday, April 15, 2013. (AP Photo/Winslow Townson)

Two Boston Marathon volunteers hug each other, at right, as an injured person is loaded into an ambulance in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Elise Amendola)

Firefighters tend to a man following an explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded at the Boston Marathon finish line Monday killing at least two people injuring dozens. (AP Photo/Michael Dwyer)

One of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon is investigated by a person in a protective suit in the wake of two blasts in Boston, Monday, April 15, 2013. (AP Photo/Elise Amendola)

Firefighters gather one block from Boylston Street following an explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Two bombs exploded at the Boston Marathon finish line Monday killing at least two people injuring dozens. (AP Photo/Michael Dwyer)

Boston Police Commissioner Edward Davis speaks about the explosions during a press conference as Massachusetts Gov Deval Patrick, right, looks on, Monday, April 15, 2013, in Boston. Two explosions disrupted the Boston Marathon on Monday, causing at least two deaths and dozens of injuries and scattering crowds near the finish line, authorities said. (AP Photo/Bizuayehu Tesfaye)

Debris is spread along the sidewalk of one of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon in the wake of two blasts in Boston Monday, April 15, 2013. (AP Photo/Elise Amendola)