Whether this event was deliberate or not is still an open question. You will be able to tell from the narrative. If they don't have a tidy story with all perpetrators dead or in no-press-access custody in a few days it was probably a 'real' 'terrorist' act.
Regardless they will try to rush through some expansion of the police state, and a complicit media will whip up the populace into a cringing fearful froth to 'demand' something must be done.
If it is a real event no one will connect the 'moral' outrage with the events like when 'we' blew 120 Afghans attending a wedding to bloody pieces. Oops. Our bad. But why the hate? You guys should be used to it by now.
So while it might be thought by some that there would be some actual motivation for a bombing in Boston, we must always remember it has nothing to do with their dead at our hands, the real reason is of course they hate us for our freedom.
(EDIT: I should add another likely scapegoat is teabaggers or other 'people who take the Constitution literally' types as per the SPLC)