I doubt it AYG. I think I know where it is though - beside the Crate and Barrell shop on the other side of Copley Square. Copley Square has some quite major hotels around it. If someone is going to bomb it, that would be the area to go for (a bit like Oxford St). It could be Al Quada linked but if it is, my guess would be that the bombers are small-fry sympathisers
Difficult to say, just about any group would use what was commonly known in the Forces as 'dockyard confetti' in a bomb, depends on the intended target.
Could be AQ, could be white supremacist nutters or even someone with sympathies to the gun lobby, given that debate is currently underway.