Boston Marathon

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Mobility-impaired runners gather at the start line for a moment of silence before the 118th Boston Marathon Monday, April 21, 2014 in Hopkinton, Mass. (AP Photo/Stephan Savoia)

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A member of the Massachusetts Department of Fire Services Haz-Mat crew affixes a device to the back of a street sign in front of Wellesley College before the start of the 118th Boston Marathon Monday, April 21, 2014 in Wellesley, Mass.. (AP Photo/Mary Schwalm)

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Quincy, Mass., police stand near the starting line of the 118th Boston Marathon Monday, April 21, 2014 in Hopkinton, Mass. (AP Photo/Michael Dwyer)

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A military policeman stands near the start line of the 118th Boston Marathon Monday, April 21, 2014 in Hopkinton, Mass. (AP Photo/Michael Dwyer)

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Buses carrying runners arrive at sunrise in Hopkinton, Mass., for the start of the 118th Boston Marathon Monday, April 21, 2014. (AP Photo/Michael Dwyer)