I'm not going to get into the debate above, but you all seem to be ignoring the fact that this bomb seems to have been designed to cause Injuries, not deaths. Permanent, debilitating ones.
Looking at one of the videos from the finish line, the the moment the bomb explodes you can see a line where stuff hits the fabric barrier's near the bottom, and a runner on the track falls over and grabs their legs. There's also a lot of reports about amputations of legs and feet. It seems like the bomb may have been designed to project shrapnel out in a disc shape at lower leg level, eg it was designed specifically to injury people.
But that's just my casual observation.
A blast pattern like that would be ridiculously difficult to produce. Most likely the devices were just packed full of ball bearings, nails etc. Those will do terrible tissue damage but the chance of a hit is reduced very fast when moving further away from the point of detonation.
A couple of things occurred to me (and please correct me if I'm wrong or forgetting something):
1) I can't remember any al-Qaeda franchise targeting an event like this. The place was swarming with cops and National Guard. From Bali to New York and London, al-Qaeda inspired hits have shunned high security targets and gone for every day concentrations of people; subways, busses, discos etc.
2) I can't remember any lone wolf-terrorist using more than a single device in a single location. The risk of getting spotted goes up with every device you leave.
3) The target really isn't something any domestic US extremists would obviously be interested in. Even the Olympic Park bombing back in the 90's had the logic of attacking a symbol of "internationalism" but the Boston marathon is a very "American" event that seems to lack strong ties to the federal government, race-issues, abortion, religion or any other thing that riles up local extremists.
In conclusion, I'm really puzzled.