I consider it progress that the MSM has not automatically decided that Islamic jihadists, Arabs or other Muslims, home-grown or foreign, are the obvious perpetrators on the horrific events in Boston. This may, of course, turn out to be the case. But, far-right wing Americans are as likely to be implicated. Are there greater probabilities for one kind of perpetrators more than another?
Both groups have been willing to target innocent civilians, but right-wingers do tend, with the exception of the Atlanta Olympics bombing, to attack symbols of government, as in the Oklahoma City bombing, or specific officials (in government or abortion providers, or threaten to do so. The only link to government in Boston, however, seems not to be a physical structure or representatives of government (e.g., police, politicians), but the choice of April 15, the day income taxes are due(attacks on taxation as a form of tyranny is a popular trope of the far-right) as well as the celebration of Patriot's Day in Boston.
Muslim terrorists could certainly find the Patriot's Day symbolism appealing and the marathon providing mass crowds and media as well.
Boston is a liberal-leaning city and one might imagine right-wing terrorists might prefer to kill those who voted for Obama and Governor Patrick, even if they are largely white, but Oklahoma City and Atlanta showed that the far right, like Muslim terrorists, are not that selective when it comes to civilians.
So, I just don't think one political tendency is more likely to have done this. However, I do think that the local police authorities and self-appointed citizen-vigilantes, especially in light of reports of a dark-skinned or black male with a foreign accent trying to enter a restricted area five minutes before the explosions will act on their implicit and explicit prejudices to question, harass, rough up and do violence to those fitting the profile of someone who have had nothing to do with the bombings. Virtually all, if not all.
Boston has a history of this. Back in 1989, Charles Stuart, a white man, killed his wife and then blamed it on a black carjacker. Police terrorized young black males in the area before evidence pointed to Stuart and he subsequently killed himself. One hopes that the memory of the Stuart case will lead to restraint by police and civilians, but the trauma in this case is city-wide and the need to catch the bomber(s) so urgent that I have doubts. If I fit the profile I would stay at home until the authorities either capture the perpetrators or identify them.