Thoughts come to one about "blowback":
*Did a pissed off "patriot" and gun owner do it? It was the official Patriots Day in Boston, celebrating April 19, 1776 first battles for Independence from the British Imperial Oppressors. It's also the 20th anniversary of the day FBI tanks started at least one fire that killed 76 Davidians in Waco, Texas as the end of a 51 day siege after ATF's violent and botched serving of a warrant for illegal guns.
*Did a pissed off national of some country the United States has bombed or has threatened to bomb the hell out of do it?
*Did a pissed off Muslim from some other country pissed off at US Imperial Oppression and what they see as the US war on Islam who has become a radical jihadist do it?
*Did a pissed off individual of whatever country pissed off at US Imperial Oppression who got manipulated by the FBI into planting the bombs - and got away from his handlers - do it?
*Did an agent of a foreign government trying to get benefits from the United States government do it? (Think especially North Korea and Israel.)
Well, if I think of any more, I'll add them. Uploading fast. More analysis by me to follow - and probably lots of conspiracy theories by others on the World Wacky Internets.
Update 4/20/13: I was right, of course. According to Washington Post (which sometimes gets it right): "Michael Newcity, a Duke University Slavic and Eastern European studies professor...[said that] the Chechen heritage of suspects Dzhokhar A. Tsarnaev and Tamerlan Tsarnaev may matter mostly because of their religion and not their birthplace... a more likely motivation was generalized anger over U.S. actions and attitudes towards Islam."
Not to mention the older brother had been questioned (probably disrespectfully) by the FBI branch of the surveillance state in 2009, couldn't find a job for last year or so because of statist destruction of the economy and was at some point a "house husband", and - of course - he was just a macho guy who had to prove his manhood (by initiating force for his religion, when boxing just didn't do it for him any more). And his dumb little brother went along with it and will spend the rest of his life in prison for a dastardly act that does no one any good except politicians and government contractors out to increase their power and politics. Don't you get it guys? Doing violence is doing the job of the government. Cut it out!