White House

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President Obama turns to leave after speaking in the Brady Press Briefing Room of the White House in Washington on April 16, 2013, following the explosions at the Boston Marathon. (Associated Press)

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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)

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** FILE ** President Obama finishes speaking in the Brady Press Briefing Room of the White House in Washington on April 16, 2013, about the Boston Marathon explosions. (Associated Press)

The American flag is lowered at the White House in Washington on April 16, 2013, honoring the victims of the tragedy in Boston. President Obama ordered flags at the White House and all government buildings to be flown at half-staff in honor of the Boston Marathon explosion victims. (Associated Press)

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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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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 Obama speaks in the James Brady Press Briefing Room at the White House in Washington, April 15, 2013. (Associated Press)

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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 have moved visitors to the north lawn of the White House back across the street as a precaution after bombs explode at the finish line of the Boston Marathon, Washington, D.C., Monday, April 15, 2013. (Andrew Harnik/The Washington Times)

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Police block off Pennsylvania Ave., from pedestrian traffic in front of the White House as a precaution after bombs explode at the finish line of the Boston Marathon, Washington, D.C., Monday, April 15, 2013. (Andrew Harnik/The Washington Times)

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An officer can be seen patroling the roof of the White House after bombs explode at the finish line of the Boston Marathon, Washington, D.C., Monday, April 15, 2013. Police have moved visitors to the north lawn of the White House back across the street as a precaution. (Andrew Harnik/The Washington Times)

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An officer can be seen patroling the roof of the White House after bombs explode at the finish line of the Boston Marathon, Washington, D.C., Monday, April 15, 2013. Police have moved visitors to the north lawn of the White House back across the street as a precaution. (Andrew Harnik/The Washington Times)

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Students from Bolder, Colo. stand behind police tape after police block off Pennsylvania Ave., from pedestrian traffic in front of the White House as a precaution after bombs explode at the finish line of the Boston Marathon, Washington, D.C., Monday, April 15, 2013. (Andrew Harnik/The Washington Times)