What would be really fucked up, is if the person who did this is a Tea Partier. All this hatred is going to simmer for Muslims* AND the government and then it comes out that this is an American who did this, but a Teapublican who did it -for- the country as a protest?
Right now we can't point to any one country with certainty, since we know that the intelligence community isn't giving out hints. We can tell they aren't giving leads because there are no buzz words filtering into the major media outlets from anywhere, and there isn't anything on social media that isn't completely made up. I mean, Obama killed Osama bin Laden, in the age of social media, and had enough control to ensure that he personally delivered the news to America instead of waiting and letting it leak. Whoever did this, if it was part of a foreign group, will only be named in the barest sense of the word. If he can get away with it, they might not ever be named so that it undermines their credibility.
Imagine if 9/11 had happened, with all the money funneled to Osama bin Laden, and 8 years later his death meant that the person who bombed the World Trade Center in '93 had been killed but darn it, we're still looking for who blew up the twin towers. What incentive would you have to attack America if they won't even give you direct credit for it? Your financial backers, who are rich but not "American" rich, won't be able to trust you if your one of 10 people who can credibly take credit for such an attack but can't prove it. Now this is a pipedream, since Americans are fanatical about getting an answer: they just don't really care so much if its 100% truthful.
So what happens then, when some Blond haired blue eyed Tea Party crazy who's motivation appears to be to try and start a second revolution since the first one started in Boston is paraded around on TV? If that happens, after weeks of propaganda and outright brainwashing by the far right wing news, whats going to happen to the peoples who well and truly trust in them? Will some of them change? Probably not. Will they double down on derp when regulations are tightened so that it's harder to make very basic IEDs? Only if it involves banning guns that can kill 20 people in the span of a minute, instead of something that requires time and patience to build and use effectively.
Now I could be wrong and it could be a terrorist from another country; but what if it's an American who thought he would be a patriot by killing other American's, and he staged the first IED bombinb in America a few months after Newtown and with lots of countries in the world saying that they would destroy us if they had the power? How bad will that disfranchisement be? Or will they even notice.
* ignoring that North Korea is actually waiving a nuclear fucking missile at us right now