Authorities scour surveillance videos for clues leading to the identities of the Boston bombers.
Day 1: http://bit.ly/1TldQIe
Day 3: http://bit.ly/1Tle9TB

[This is the second of five installments of an ABC News digital special report '5 Days', which tells the hour-by-hour, day-by-day story of the investigation into the 2013 Boston Marathon bombing, with behind-the-scenes details based on interviews and evidence gathered by the FBI and Boston law enforcement .]

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