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Police officers search house to house for the second suspect in the Boston Marathon bombings in a neighborhood of Watertown, Massachusetts April 19, 2013. Black Hawk helicopters and heavily armed police descended on a Boston suburb Friday in a massive search for an ethnic Chechen suspected in the Boston Marathon bombings, hours after his brother was killed by police in a late-night shootout. REUTERS/Brian Snyder (UNITED STATES - Tags: CRIME LAW) REUTERS /BRIAN SNYDER /LANDOV   Photographers/Source: BRIAN SNYDER/Reuters /Landov
Police officers search house to house for the second suspect in the Boston Marathon bombings in a neighborhood of Watertown, Massachusetts April 19, 2013. Black Hawk helicopters and heavily armed police descended on a Boston suburb Friday in a massive search for an ethnic Chechen suspected in the Boston Marathon bombings, hours after his brother was killed by police in a late-night shootout. REUTERS/Brian Snyder (UNITED STATES - Tags: CRIME LAW) REUTERS /BRIAN SNYDER /LANDOV   Photographers/Source: BRIAN SNYDER/Reuters /Landov

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Tracking down the killers

John Negroponte, Jim Johnson and Candice DeLong on tracking down the killers in the Boston bombings.

Tracking down the killers

John Negroponte, Jim Johnson and Candice DeLong on tracking down the killers in the Boston bombings.