Boston Marathon

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The flag is lowered half staff above the White House in Washington, Tuesday, April 16, 2013, following the explosions at the Boston Marathon. (AP Photo/Charles Dharapak)

British sports minister Hugh Robertson, shown here in a July 26, 2011 file photo, said Britain "won't be cowered by this sort of behavior" in announcing that Sunday's London Marathon will go on in the wake of Monday's bombings at the Boston Marathon. (AP Photo/Sang Tan)

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Martin Richard, 8, was killed in the explosion at the Boston Marathon. (Twitter photo)

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President Obama speaks in the James Brady Press Briefing Room at the White House in Washington on Monday, April 15, 2013, following the explosions at the Boston Marathon. (Associated Press)

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Members of the St. Louis Cardinals line up for a moment of silence following an explosion at the Boston Marathon, all wearing the No. 42 in honor of Jackie Robinson, before a baseball game against the Pittsburgh Pirates in Pittsburgh Monday, April 15, 2013. (AP Photo/Gene J. Puskar)

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President Barack Obama arrives to speak 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)

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** FILE ** President Barack Obama speaks 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/Manuel Balce Ceneta)

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President Obama leaves the podium after speaking in the James Brady Press Briefing Room at the White House in Washington on April 15, 2013, following the explosions at the Boston Marathon. (Associated Press)

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Police officers react to a second explosion at the finish line of the 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 Boston Globe, John Tlumacki)

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Emergency responders aid a woman on a stretcher who was injured in a bomb blast near the finish line of the Boston Marathon Monday, April 15, 2013 in Boston. Two bombs exploded in the packed streets near the finish line of the marathon on Monday, killing at least two people and injuring more than 80, authorities said. (AP Photo/Jeremy Pavia)

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** FILE ** 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)