A terrorism expert has said he thinks the bombs at the Boston Marathon are the work of "either an Al Qaeda-affiliated group or a right-wing extremist group."
Dr Max Abrahms at Johns Hopkins University in Baltimore said the explosions could have been the result of foreign or domestic terrorists.
Al Qaeda has become decentralised because of years of US action against it, which meant it could be behind small attacks like this, he told Sky News.
But he also said right-wing groups like those led by Timothy McVeigh, who killed 168 in the Oklahoma City Bombing in 1995, could also be behind it because of the 20th anniversary anniversary of the end of the Waco siege, in which 76 people died, on April 19.
"This was around the time of year when we might expect to see the Timothy McVeigh-type bomber come out of the woodwork because it is the anniversary of Waco, when the United States invaded a large group of these types of sympathisers," he told the broadcaster.
"I think it is either an Al Qaida-affiliated group or a right-wing extremist group."
At least three people have been killed and as many as 140 people have been injured after two explosions at the marathon
A law enforcement official said the bombs appeared to have been made with black powder and ball bearings, but as yet did not know how the explosions were triggered.
Boston is the world's oldest marathon and takes place to coincide with Patriots' Day every year, the third Monday in April, which commemorates the anniversary of the first battles of the American Revolutionary War.
The spectacle regularly attracts more than 20,000 participants and 500,000 spectators each year and the final mile of this year's event was dedicated to victims of Newtown shootings.
Police have said no suspect yet has been arrested in connection with the shootings, amid reports that a student from Saudi Arabia had been taken in custody.
Law enforcement officials said the man was questioned because of his injuries, his proximity to the attack and because of his visa, but said he was not known to authorities.