It could be Al-Qaeda, via the Dallas Morning News:

A trademark of al-Qaeda and its affiliates is to launch attacks in ways that are highly photogenic, guaranteed to inflict maximum casualties and, most important, generate maximum television coverage. Something like 27,000 people run in the Boston Marathon, and thousands more gather along the route, with heavy concentrations at the finish line. The fact that multiple explosives have been found near the Boston Marathon site suggests another trademark of al-Qaeda, which has a preference for attacks in multiple stages, or multiple bombs set to go off all at once in a big blast.

Don't blame Al-Qaeda first, blame right wing extremists, via Esquire:

Obviously, nobody knows anything yet, but I would caution folks jumping to conclusions about foreign terrorism to remember that this is the official Patriots Day holiday in Massachusetts, celebrating the Battles at Lexington and Concord, and that the actual date (April 19) was of some significance to, among other people, Tim McVeigh, because he fancied himself a waterer of the tree of liberty and the like.

It could've been some kind of inside job, via Foreign Policy:

The fevered ravings on Twitter are diverse: The bombs were planted by the U.S. government, or by Mossad agents, or by conspiracy theory bogeyman the Illuminati! To drive up the price of gold and silver! To justify new gun control laws! Or war in Syria! Or Iran! Or North Korea! Or, based on the timing (today being Tax Day and Massachusetts's "Patriots's Day," commemorating the battles of Lexington and Concord in the Revolutionary War), to crack down on right-wing and libertarian groups!

2013, when all the news can fit your views.

The National Journal provides advice that shouldn't be necessary to reporters, observers and even police (see: JFK electric fire), but sadly is: it's too early to jump to conclusions.

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