Since nobody has claimed responsibility for the atrocity (as terrorist groups usually do), I tend to think it's a local nutcase behind the bombings.
I also think that bb's were used in place of ball bearings since they are cheap and easily obtainable.
3 dead, 17 critical, and 176 injured total. I hope they solve this horrendous atrocity soon.
Officials: No unexploded devices found | Boston Herald