It is just my theory, but the ball bearing signature gives a strong indication of who is behind the bomb making. Most domestic bombmakers would use nuts or nails from Home Depot, because they are so easily available. Ball bearings effective enough in a bomb need to follow a design that has been developed by people who know how to use them. I am not sure a novice will know how to do it right. The Islamic terrorists have lots of experience and they know what design works best. The nature of those bombs in Boston came from people who have experience making them. That experience sure paid of and that is why I suspect someone with Islamic terrorist experience made them.
The injuries inflicted also suggest that the bomb may have been a shaped charge design (like a claymore) that directed the force of the blast in a low, fan- shaped pattern. I have some projectiles from a Viet Cong anti-personnel mine that are pieces of steel reinforcing rod cut up with a bolt cutter. Those were wicked.