Boston Marathon Explosion

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, April 15, 2013.

An emergency responder and volunteers, including Carlos Arredondo in the cowboy hat, push Jeff Bauman in a wheel chair after he was injured in an explosion near the finish line of the Boston Marathon April 15, 2013.

Medical workers transport the injured across the finish line during the 2013 Boston Marathon following an explosion, April 15, 2013.

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion, April 15, 2013.

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 April 15, 2013.

Runner John Ounao cries when he finds friends after several explosions rocked the finish of the Boston Marathon, April 15, 2013.

A police officer clears Boylston Street following an explosion at the finish line of the 2013 Boston Marathon, April 15, 2013.

Medical workers aid a wounded woman at the finish line of the 2013 Boston Marathon following two explosions there, April 15, 2013.

Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, April 15, 2013.

A woman is comforted by a man near a triage tent set up after explosions went off at the 117th Boston Marathon, April 15, 2013.

A Massachusetts state police officer guards the area containing the medical tent, rear, following an explosion at the 2013 Boston Marathon, April 15, 2013.

An unidentified Boston Marathon runner leaves the course crying near Copley Square following an explosion, April 15, 2013.

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, April 15, 2013.

Justine Franco of Montpelier, Vermont, holds up a sign near Copley Square looking for her missing friend, April, who was running in her first Boston Marathon, April 15, 2013.

President Barack Obama leaves the podium after speaking in the press briefing room at the White House, April 15, 2013, following the explosions at the Boston Marathon.