The brown paper bag inside a white plastic bag (or whatever it was) that was in front of the metal barriers could not have been the source of the explosion, IMO, since photos of the scene post-blast show the barriers bent toward the street. If the blast had originated in front of the barriers, I think it would have blown the metal barriers backwards, away from the street.
Based on photographic evidence, the explosion clearly came from behind the metal barriers, since the metal barriers were bent toward the street post-blast.
Pre-blast photo:
Post-blast photo:
http://www.mirror.co.uk/news/world-news/boston-marathon-bomb-photo-shows-1836532
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