When it comes to the events in Boston, my guess is that it's US domestic terrorism and more likely by an individual than a group.

If it would have been by let's say a group claiming al-Qaeda affiliation, an announcement of responsibility would have been ready (at some point one can assume all kinds of groups will claim that they did it, but the longer it takes to make the claim, the less weight one can put on such claims).

Al-Qaeda affiliates also tend to make their plans over-complicated, which makes easier to find and stop them in time. This appears to have been simple and practical, and thus much easier to pull off than your typical al-Qaeda affiliate plot.

On the US' domestic front, individuals tend to slip more easily pass the government agencies' watch than groups as there is naturally less loose ends that the authorities can find and track. Just think of the Spokane bombing attempt a few years back.

Overall, FBI etc should spend less time grooming disaffected individuals so that they can make headlines for a few days when FBI announces the "plot" it recruited them to take part in, and then spend decades in jail, and concentrate on real threats instead.

Southern Poverty Law Center could offer more than a few tips for the FBI for real targets to monitor.