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BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) A man comforts a victim on the sidewalk at the scene of the first explosion near the finish line of the 117th Boston Marathon. (Photo by John Tlumacki/The Boston Globe via Getty Images)
BOSTON - APRIL 15: (EDITOR'S NOTE: THIS IMAGE CONTAINS GRAPHIC CONTENT) A man comforts a victim on the sidewalk at the scene of the first explosion near the finish line of the 117th Boston Marathon. (Photo by John Tlumacki/The Boston Globe via Getty Images)

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McCaul: Gun powder likely used in bombs

Erin Burnett talks to Homeland Security Chmn. Mike McCaul about new details in the Boston terror attack.