If I had to make a bet, I would guess this is domestic. However, it's too early and there's too little information to do more than guess.
This
Yesterday I was thinking for the better part of the day "I bet it's some nutbar teabagger" before realizing how much of a frothing lunatic I would look like if it wasn't. Not to mention how much of an ideological hack I'd be. It's not exactly our job to speculate nor should we because then it may become us ascribing horrible positions to those we don't like, thus politicizing this tragedy. The police (local and federal) are on the case and I can only hope we know the responsible party soon enough.
That said, I do think it was some body who acted alone and somebody domestic.
While it might not be a tea partier, someone on here got me thinking that it might be similar to that guy who tried to kill IRS agents a few years ago.
--Patriot Day (uniquely celebrated in Boston)
--Tax Day (they fell on the same day this year)
--Congress is debating tighter gun restrictions
These three things suggest to me that a "Tea Party Type" (although 'not' necessarily someone who IDs with the TP themselves - I'm using it as more of a lazy identifier) might have been behind this. Of course that's speculation and we need more concrete proof but the collusion suggests to me that this was a lone nutter, or if he had help, a very small group of crazies, who styled themselves as "good patriots" (ala the tea party - hence the lazy identifier) who were resisting a tyrannical government and staged an event as some wackadoo form of protest.
Also, and this just came to me, if you wanna get a 'bit' more conspiratorial, it's possible that while frothingly crazypants, at least one of them was smart (or maybe stupid) enough to realize a massive portion of the country 'would' try to "Blame the Arab" or "Blame the North Korean" with the latter being more likely.. since even highly intelligent people like my mom were asking "I wonder if it's North Korea" before agreeing how impossible that'd be when I explained how difficult it would be for a national to get into the US, plant bombs, and detonate them without getting caught, with how many eyes we have on NK right now.
No, there needs to be more evidence before anything concrete can be claimed... but as far as Sherlock-y speculation goes.. anti-government nutter, anti-gun nutter, or regular nutter, working either alone or in a small group. IANACP