The FBI was reportedly in contact with one of the Boston bombing suspects, raising questions about whether the attack which killed three people at the marathon could have been prevented.
US media reported that the FBI question Tamerlan Tsarnaev in 2011 because suspicions had been raised by a foreign government - thought to be Russia - that he was a radical Islamist.
The bureau found no evidence of any suspicious activity, after a "sit-down" interview with Tsarnaev, checks of travel records, internet activity and personal associations.
The FBI said in a statement: “The request stated that it was based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country’s region to join unspecified underground groups.
"In response to this 2011 request, the FBI checked U.S. government databases and other information to look for such things as derogatory telephone communications, possible use of online sites associated with the promotion of radical activity, associations with other persons of interest, travel history and plans, and education history.
"The FBI also interviewed Tamerlan Tsarnaev and family members. The FBI did not find any terrorism activity, domestic or foreign, and those results were provided to the foreign government in the summer of 2011.
"The FBI requested but did not receive more specific or additional information from the foreign government."
The Telegraph reported a US law enforcement source saying no “derogatory” information could be found about Tamerlan, and the matter was put “to bed”.
Congressman Peter King, former chairman of the House Homeland Security Committee, told Newsday he fears the FBI might have missed something. "Did they move too quickly by letting this guy off the hook?" King said in an interview. "Should they have looked more carefully?"
King said he and Homeland Security Committee chairman Michael McCaul are drafting a letter to US Attorney General Eric Holder and FBI Director Robert Mueller, requesting a classified briefing on the 2011 interview with Tsarnaev.
King said he wants to know "what information was given and what was done with it."
Tsanaev is reported to have the visited the North Caucuses, including Dagestan and Chechnya, for a six month visit last year, according to the Guardian.
Zubeidat Tsarnaeva, the mother of the two boys, told Russia Today she believed her sons were under surveillance for years.
She said in an interview with the station: “They used to come [to our] home, they used to talk to me…they were telling me that he [the older, 26-y/o Tamerlan] was really an extremist leader and that they were afraid of him.
"They told me whatever information he is getting, he gets from these extremist sites… they were controlling him, they were controlling his every step…and now they say that this is a terrorist act! Never ever is this true, my sons are innocent!”
More than 50 victims remain in hospital, with three critically ill.
Not far from the scene of the police shootout in Watertown that killed his older brother, the second suspect Dzhokhar Tsanaev, was discovered hiding in a boat. He was severely wounded and taken to hospital after a shootout that lasted around an hour.
Police have said he is in a "serious condition" after suffering significant blood loss, and cannot speak because of a throat injury.