Hi j-t,
I've ruled out foreign terrorists in my hypotheticals.
The only foreign organisation we know about that does this is AQ.
In my view, the organisation that undertook 9/11 and the London underground bombing would not bother with the Boston Marathon event. It was too small scale for those guys. And it didn't attack an important source of power. Just doesn't have AQ proper's MO, in my view.
Home-grown, I reckon. So then you look for nasty guys who make bombs. And lo and behold, there's an active group out there.
I work back from motive. There is always a purported justification.
The right wing in America is very capable of doing this sort of stuff. They are always being whipped into a frenzy of self-righteousness. Only listen to Limbaugh and his ilk.