In addition to domestic law enforcement, intelligence agencies such as the CIA and the National Counterterrorism Center have joined the investigation.

At least two, essentially contradictory, theories about who might be behind the explosions were under immediate consideration by investigators, officials told Reuters on the condition of anonymity.

One theory, based at least in part upon the timing of the bombings—on the day when U.S. tax returns are due and also on the day of the Boston Marathon, which is customarily held on Patriots' Day—implies that domestic right-wing extremists, such as anti-government anti-tax activists, could be behind the attacks.

One of the main alternative theories, which was commended to Reuters by senior law enforcement officials, is that there could be some kind of connection, whether direct or very loose, to Islamic militants.

Supporting the second theory: the extremist magazine Inspire, according to Reuters, recently published "a lengthy supplement" encouraging people to attack high-profile events using homemade devices.