10ThingstoSeeSports - A crowd gathers at the finish line of the Boston Marathon in Boston for a Sports Illustrated photo shoot before the one-year anniversary of the Boston Marathon bombings, Saturday, April 12, 2014. (AP Photo/Michael Dwyer, File)
Boston Police officer John Quinn walks with, Miller, his bomb detection canine, over the finish line while sweeping the area in preparation for the Boston Marathon, Wednesday, April 16, 2014, in Boston. (AP Photo/Charles Krupa)
Massachusetts Emergency Management Agency Director Kurt Schwartz speaks with the media following a news conference in preparation for the Boston Marathon, Wednesday, April 16, 2014, in Boston. (AP Photo/Charles Krupa)
Boston Police Commissioner William Evans speaks during a news conference in preparation for the Boston Marathon as Massachusetts Emergency Management Agency Director Kurt Schwartz, right, listens, Wednesday, April 16, 2014, in Boston. (AP Photo/Charles Krupa)
Boston Police Commissioner William Evans, second from left, listens to Boston Marathon Race Director Dave McGillivray during a news conference in preparation for the Boston Marathon, Wednesday, April 16, 2014, in Boston. (AP Photo/Charles Krupa)
Boston Marathon Race Director Dave McGillivray gestures during a news conference in preparation for the Boston Marathon, Wednesday, April 16, 2014, in Boston. (AP Photo/Charles Krupa)
A member of the bomb squad inspects an object after a controlled detonation at the finish line of the Boston Marathon in Boston, Tuesday, April 15, 2014. Police have blown up two unattended backpacks found near the Boston Marathon's finish line on Tuesday. They say they've taken a man into custody in connection with them. (AP Photo/Michael Dwyer)
A member of the bomb squad walks toward the Boston Marathon finish line before triggering a controlled detonation in Boston, Tuesday, April 15, 2014. Police have blown up two unattended backpacks found near the Boston Marathon's finish line on Tuesday. They say they've taken a man into custody in connection with them. (AP Photo/Michael Dwyer)
Police confer near the finish line of the Boston Marathon in Boston before the second of two controlled detonations, Tuesday, April 15, 2014. Police have blown up two unattended backpacks found near the Boston Marathon's finish line on Tuesday. They say they've taken a man into custody in connection with them. (AP Photo/Michael Dwyer)
A Boston policeman watches from behind a fire truck before a controlled detonation at the Boston Marathon finish line in Boston, Tuesday, April 15, 2014. Police have blown up two unattended backpacks found near the Boston Marathon's finish line on Tuesday. They say they've taken a man into custody in connection with them. (AP Photo/Michael Dwyer)