This, along with the fact that nobody has claimed responsibility leads me to think the same.
That, and it was Boston. Birthplace of American Liberty, Boston Massacre, "no taxation without representation", Tea Party, Radical Revolutionaries, etc. etc. etc. It's where it all began. Some extremist militia type sees this as striking a blow for "patriotic 'muricans ever where".
I know the FBI has most of these type groups infiltrated or under careful watch. What's scary is that they obviously had no idea this one was coming. The (good) problem with Joe Bubba militias is that some redneck gets drunk down at the pool hall and blabs about the big plan to blow shit up, and they all get busted before you can say "Pabst Blue Ribbon". But not this time. This has the fingerprints of a carefully planned militaristic operation all over it. Multiple devices, delayed sequential detonations, and complete mission secrecy. I would wager no more than three guys - more than that and loose lips or an attack of conscience will sink ships. This was professional. I am going to guess someone spent some time somewhere in a sandbox and then came home with new skills and a bad attitude.