A few things that the investigators are almost certainly working on:
- April 15: Tax Day, hated by many far-right-wing anti-government extremists
- Patriot's Day: another potential "inspiration" if this were, in fact, a right-wing "Patriot" whackjob
- Boston: where the Tea Partiers got their inspiration, from the "patriots" who took part in the Boston Tea Party
- Massachusettes has fairly tough gun registration laws
- The race was dedicated to the victims of the Newtown shootings, which instigated nation-wide calls for effective gun control
- Massachusettes has a black governor
- The US has a black President
I also noticed that during the second, prime-time special on NBC, anchor Bryan Williams said, "For all the wrong reasons, we've been fighting two wars for the past ten years," referring to the fact that many returning combat medics have seen these horrific injuries before.
My personal bet is some far-right-wing whacko like Eric Rudolph, Jared Loughner, McVeigh... One of those losers.