In a further twist, Channel 4 News claimed on Sunday that Tamerlan had phoned home in the wake of the bombings and told his mother that the FBI had already called him to accuse him of being responsible.

According to the report, Tamerlan telephoned his mother on Wednesday last week and said he had replied to the FBI’s accusations by saying : « That’s your problem ». The claims emerged from an interview with his father, Ansor.

The FBI declined to comment directly on either claim on Sunday.

However, if either is confirmed, they will add significant weight to the growing chorus of criticism of the FBI, which came on Sunday from senior members of Congress who accused the Bureau of repeatedly « dropping the ball ».

Michael McCaul, the chair of House Homeland Security Committee, said the FBI must explain why it failed to keep track of Tsarnaev after the 2011 interview, particularly after he visited his family in Dagestan, which is a known centre of Islamist militancy and training facilities.

« If he [Tamerlan Tsarnaev] 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 ? » Mr McCaul asked on CNN, adding that there were clear signs that Tsarnaev had been radicalized during his trip.

« 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 radicalized at some point in time, » Mr McCaul added. « Where was that and how did that happen ? »

In a further sign that Tsarnaev’s record was in the US security apparatus, the New York Times reported yesterday that a « hold » had been placed on his citizenship request by the Department of Homeland Security after routine background checks discovered the FBI’s former interest in him.

The FBI has also not explained why it did not immediately retrieve the Tsarnaev file after the bombs exploded on Monday afternoon – an event which should have triggered routine checks on those suspected of involvement in with Islamist militant groups.