So far the Tea Party hasn't really been involved in any violence so assuming their involvement seems desperate. That being said this being tax day, the target being Boston, and it being Patriot's Day could indeed suggest domestic terrorism. It sounds like the this could have been a lot worse, but multiple bombs didn't go off and the big one detonated at a period where there weren't many people around (relatively, compared to the busier moments of the marathon).
Conflicting reports from the JFK library too. Eh