Police stand guard outside the United Center with heightened security measures in effect due to the bombings in Boston before the start of the NHL hockey game between the Dallas Stars and the Chicago Blackhawks in Chicago, April 15, 2013. [Photo/Agencies]

SWAT officers stand guard after two explosions interrupted the running of the Boston Marathon in Boston, Massachusetts April 15, 2013. Two bombs ripped through the crowd at the finish line of the Boston Marathon on Monday, killing two people and injuring dozens in what a White House official said would be handled as an "act of terror." [Photo/Agencies]