Boston Marathon bombing

Still image taken from video courtesy of NBC shows an explosion at the Boston Marathon, April 15, 2013. Two explosions struck the marathon as runners crossed the finish line on Monday, witnesses said, injuring an unknown number of people on what is ordinarily a festive day in the city. REUTERS/NBC/Handout (UNITED STATES - Tags: CRIME LAW SPORT ATHLETICS) FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. NO ARCHIVES. CNN OUT. AOL OUT. YAHOO OUT

Runners continue to run towards the finish line of the Boston Marathon as an explosion erupts near the finish line of the race in this photo exclusively licensed to Reuters by photographer Dan Lampariello after he took the photo in Boston, Massachusetts, April 15, 2013. Two simultaneous explosions ripped through the crowd at the finish line of the Boston Marathon on Monday, killing at least two people and injuring dozens on a day when tens of thousands of people pack the streets to watch the world famous race. REUTERS EXCLUSIVE Mandatory Credit REUTERS/Dan Lampariello (UNITED STATES - Tags: CRIME LAW SPORT ATHLETICS) MANDATORY CREDIT

BOSTON - APRIL 15: A second explosion goes off (rear) as a runner was blown to the ground by the first explosion near the finish line of the 117th Boston Marathon. (Photo by John Tlumacki/The Boston Globe via Getty Images)

BOSTON - APRIL 15: Police officers with their guns drawn hear the second explosion down the street. The first explosion knocked down a runner at the finish line of the 117th Boston Marathon. (Photo by John Tlumacki/The Boston Globe via Getty Images)

BOSTON - APRIL 15: A second explosion goes off near the finish line of the 117th Boston Marathon on April 15, 2013. (Photo by David L. Ryan/The Boston Globe via Getty Images)

Police and runners react to an explosion during the Boston Marathon finish area in Boston, Massachusetts, April 15, 2013. Two simultaneous explosions ripped through the crowd at the finish line of the Boston Marathon on Monday, killing two people and injuring dozens on a day when tens of thousands of people pack the streets to watch the world famous race. REUTERS/MetroWest Daily News/Ken McGagh/Handout (UNITED STATES - Tags: CRIME LAW SPORT ATHLETICS TPX IMAGES OF THE DAY) FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. NO ARCHIVES. MANDATORY CREDIT

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

BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) Injured people lie on the sidewalk near a barrier at the scene of the first explosion that went off near the finish line of the 117th Boston Marathon. (Photo by John Tlumacki/The Boston Globe via Getty Images)

BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) Bystanders help an injured woman at the scene of the first explosion on Boylston Street near the finish line of the 117th Boston Marathon. (Photo by John Tlumacki/The Boston Globe via Getty Images)

Medical responders run an injured man past the finish line 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)

epa03663348 A couple embrace near the finish line area after reports of two explosions near the finish of the 117th Boston Marathon in Boston, Massachusetts, USA 15 April 2013. EPA/CJ GUNTHER

BOSTON - APRIL 15: A man lays on the ground after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (Photo by David L. Ryan/The Boston Globe via Getty Images)

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)

BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) A person who was injured in an explosion near the finish line of the 117th Boston Marathon is taken away from the scene in a wheelchair. (Photo by David L. Ryan/The Boston Globe via Getty Images)

BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) A person who was injured in an explosion near the finish line of the 117th Boston Marathon is taken away from the scene on a stretcher. (Photo by David L. Ryan/The Boston Globe via Getty Images)

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 near the finish of the Boston Marathon on Monday, killing at least two people, injuring over 20 others. (AP Photo/Charles Krupa)

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

BOSTON - APRIL 15: Women react at Boylston Street and Massachusetts Avenue, frustrated not being able to reach loved ones following explosions near the finis line of the Boston Marathon. (Photo by Bill Greene/The Boston Globe via Getty Images)

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)

Runner John Ounao crying when he finds friends after several explosions rocked the finish of the Boston Marathon in Boston, Massachusetts, on April 15, 2013. At least two people were killed and 22 wounded when two explosions struck near the finish line of the Boston Marathon, sparking scenes of panic, police said. The streets were littered with debris and blood and paramedics raced off with stretchers as police locked down the area, witness said. TV footage showed an explosion sending up a white plume of smoke along the sidelines of the race. AFP PHOTO/John MOTTERNJOHN MOTTERN/AFP/Getty Images

BOSTON, MA - APRIL 15: A runner embraces another woman on the marathon route near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

This photo provided by Bruce Mendelsohn shows the scene after two explosions occurred during 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/ Bruce Mendelsohn) MANDATORY CREDIT

BOSTON - APRIL 15: A woman kneels and prays at the scene of the first explosion on Boylston Street near the finish line of the 117th Boston Marathon on April 15, 2013. (Photo by John Tlumacki/The Boston Globe via Getty Images)

BOSTON - APRIL 15: Runners who had not finished the race were stopped before the Massachusetts Avenue overpass on Commonwealth Avenue after two explosions went off near the finish line of the 117th Annual Boston Marathon in Boston, Mass. on Monday, April 15, 2013. (Photo by Yoon S. Byun/The Boston Globe via Getty Images)

BOSTON - APRIL 15: Two men look under bleachers for another device after two explosions went off near the finish line of the 117th Boston Marathon on April 15, 2013. (Photo by David L. Ryan/The Boston Globe via Getty Images)

People comfort each other after explosions went off at the 117th Boston Marathon in Boston, Massachusetts April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. REUTERS/Jessica Rinaldi (UNITED STATES - Tags: CIVIL UNREST DISASTER SPORT ATHLETICS)

Ken Poole (right) and Eric Uganecz (left) stand waiting for their loved ones to come for them following the bombing during the Boston Marathon in Boston, Massachusetts on April 15, 2013.

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)

People make phone calls to loved ones at a makeshift AT&T booth with battery chargers following the bombing during the Boston Marathon in Boston, Massachusetts on April 15, 2013.

An unidentified Boston Marathon runner, center, is reunited with loved ones near Copley Square following an explosion in Boston Monday, April 15, 2013. (AP Photo/Winslow Townson)

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)

Massachusetts Gov. Deval Patrick speaks as Boston Police Commissioner Ed Davis, middle, and FBI Special Agent in Charge Richard DesLauriers, far right, listen at a news conference in Boston Monday, April 15, 2013 regarding two bombs which exploded in the street near the finish line of the Boston Marathon on Monday, killing three people and injuring more than 130. (AP Photo/Elise Amendola)

A runner retrieves his belongings from a Boston Athletics Association worker at a sorting area near the site of an explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. (AP Photo/Michael Dwyer)

BOSTON, MA - APRIL 16: A Boston police officer stands near the scene of a twin bombing at the Boston Marathon, on April 16, 2013 in Boston, Massachusetts. Three people are confirmed dead and at least 141 injured after the explosions went off near the finish line of the marathon yesterday. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo by Spencer Platt/Getty Images)