April 17, 2013
A man looks down Boylston street towards the finish line of the Boston Marathon in Boston on April 17, 2013. The investigation of the Boston Marathon bombing is focusing on a suspect or suspects believed to have carried heavy bags or backpacks, but entered a third day on Wednesday without any arrests or word on who was responsible. REUTERS/Shannon Stapleton
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Investigators have spotted a Boston Marathon bombing suspect from security video taken before two blasts ripped through central Boston on Monday, a U.S. law enforcement source said on Wednesday, in what is potentially the biggest break in the case yet.