So far no organization or individual has claimed responsibility for the two explosions that went off at the finish line of the signature sporting event.

Investigators have not voiced any official theories as to who could be behind the attack, only saying that they're treating it as a potential act of terror.

Addressing the nation for the second time in 24 hours following the tragedy, President Barack Obama said the deadly Boston Marathon bombings “were an act of terrorism”. He said investigators do not know if they were carried out by an international or domestic organization, or perhaps by a "malevolent individual." Unofficially, two possibilities have been suggested. One points the finger at domestic right-wing radicals, who may have timed the attack with Patriots' Day in the US. Monday was also the day when tax returns are due in America, which may also see a link to anti-government extremists.

Another version put the blame on Islamist militants. Planting multiple explosive devices at a large public event or a public venue like a marketplace is a common tactic for terrorist attacks in Afghanistan, Pakistan, Iraq and Yemen. The Pakistani Taliban has denied any role in the bombings.

http://rt.com/usa/boston-blasts-investigation-evidence-921/