The US war in Iraq and Afghanistan drove the two Chechen-origin brothers to carry out the twin blasts in Boston, media reports claimed, citing authorities interrogating one of the suspects.
Dzhokhar Tsarnaev, 19, who is being treated for injuries in a hospital in Boston after he was arrested on April 19, has been communicating with federal authorities in a limited fashion.
Charged of using 'weapons of mass destruction', he faces death penalty if convicted.
According to media reports, Dzhokhar told investigating authorities that he and his elder brother Tamerlan Tsarnaev, 26, were motivated by the US war in Iraq and Afghanistan and that they had no contact with any foreign terrorist group.
Tamerlan died in a police shootout last week.
However, authorities cautioned that these were preliminary stages of the investigation and no definite conclusion can be drawn at this point.
"These are persons operating inside the United States without a nexus," to an overseas group, a US intelligence official was quoted as saying by The Washington Post.
Instead, officials said, the evidence suggests that the two brothers were "self-radicalised."
"The 19-year-old has told investigators the brothers were self-radicalised via the internet. Investigators also are looking into whether the online English-language magazine Inspire, put out by al Qaeda in the Arabian Peninsula, was used for instruction on how to make the bombs, but another source cautioned that other outlets could have provided that information," CNN reported.
Dzhokhar, The Boston Globe said, admitted that he and his brother were behind the twin blasts at marathon, which killed three people and injured nearly 200 others.
Police sources told The Boston Globe that the carjack victim said Dzhokhar and his older brother, Tamerlan, pointed guns at him and, in an apparent effort to intimidate the victim and dissuade him from trying anything foolish, Tamerlan told him, "We just killed a cop. We blew up the marathon. And now we're going to New York. Don't (expletive) with us."
Dzhokhar told investigators the attack was devised from the internet, ABC news reported.
Republican senator Marco Rubio made a similar statement after a classified FBI briefing.
"The increasing signals are that these were individuals who were radicalised, especially the older brother, over a period of time - radicalised by Islamist fundamentalist terrorists, basically using internet sources to gain not just the types of philosophical beliefs that radicalised them, but also learning components of how to do these sorts of things," Rubio told reporters.
"This is a new element of terrorism that we have to face in our country. We need to be prepared for Boston-type attacks, not just 9/11-type attacks," he said.