There could be some similarities between this bombing and the one that happened in Atlanta, GA in July, 1996.
Apparently the bomb used today was left in a pack, as was the one used in Atlanta.
Even though the BM doesn’t seem like an obvious target of right-wing terrorism, neither did the 1996 Summer Olympics.
As has been mentioned above, this is Patriot Day and Income Tax day, so I think there is a good possibility (but not a probability), that this might be the act of some anti-government or anti-tax extremist(s).
One reason I’m considering this perspective is that it makes me less fearful when I go shopping. I was looking at the possibility that if this was an act committed by “Islamic” terrorists, they could just as well hit a crowded store, such as Whole Foods, and darn if I didn’t forget to check for discarded or abandoned backpacks when I was there today. ;-o