FBI’s scrutiny of Boston video walks a privacy-security tightrope

As was expected during one of Boston’s biggest days on Monday, the presence of media cameras and smartphone-wielding spectators at the Marathon’s finish line was so common as to almost seem frivolous.

But after two bombs ripped through the crowds, killing three people and wounding at least 170 more, those photos and shaky videos, reportedly combined with surveillance footage from a nearby department store, have likely served a pivotal purpose in the investigation. Several media outlets reported Wednesday that a suspect may have been identified from surveillance video taken at a Lord & Taylor store between the two bomb blasts. And authorities have an image of a possible suspect carrying, and perhaps dropping, a black bag at the second bombing scene on Boylston Street, according to Boston.com.