The agency has already admitted that it interviewed Tamerlan Tsarnaev in 2011 after Russia raised concerns that he was becoming a follower of radical Islam, but found nothing "derogatory" against him and did not pursue the case.
His family has claimed Tamerlan went to Dagestan to visit his father, but an aunt, Patimat Suleimanova, told the Guardian that he arrived in the area before his father.
She said Tamerlan left the United States in January 2012 and arrived in Dagestan around March. His father, Anzor Tsarnaev, arrived in the republic only in May.
"He came to become acquainted with [Dagestan]," Suleimanova said. "He would sit at home and pray. He was learning to read the Koran. He saw relatives, friends."
Tamerlan, who allegedly orchestrated last Tuesday's bombings with his younger brother Dzhokhar, was killed in the early hours of Friday in a shoot-out with police. It took place just hours after detectives released images of the pair to the public.
In a further twist, Channel 4 News claimed yesterday that Tamerlan had phoned home after the bombings and told his mother that the FBI had already called him to accuse him of being responsible.
According to the report, he replied to the accusations by saying: "That's your problem."
The FBI declined to comment yesterday on either claim.
If either is confirmed, it will add significant weight to the growing chorus of criticism of the FBI.
Yesterday senior members of Congress accused the bureau of repeatedly "dropping the ball".
Michael McCaul, the chairman of the House homeland security committee, said the FBI must explain why it failed to keep track of Tamerlan after the 2011 interview, particularly after he visited Dagestan, a known centre of Islamist militancy and training facilities.
"If he was on the radar and they let him go, if he was on the Russians' radar, why wasn't a flag put on him, some sort of customs flag?" McCaul asked on CNN.
"One of the first things he does [upon his return] is puts up a YouTube website throwing out a lot of jihadist rhetoric. Clearly something happened, in my judgment, in that six-month timeframe - he radicalised at some point in time," McCaul added.
The FBI has not explained why it did not immediately retrieve the Tsarnaev file after the Boston bombs went off - an event that should have triggered routine checks on those suspected of involvement with Islamist militant groups.
Senator Lindsey Graham warned: "It's people like this that you don't want to let out of your sight, and this was a mistake. Either our laws are insufficient or the FBI failed, but we're at war with radical Islamists and we need to up our game."
-Telegraph Group Ltd
