Members of Congress ask why no flags were raised after FBI was asked to interview elder Tsarnaev brother in 2011

The FBI was facing increased scrutiny Sunday over its prior handling of a case involving suspected Boston bomber Tamerlan Tsarnaev, with senior congressional figures demanding answers and accusing the agency of "dropping the ball".

As younger brother Dzhokhar remained in hospital yet to be questioned or charged, Tamerlan – who was killed in a shootout with police early Friday – has been seen by many as the key to understanding how the pair seemingly became radicalised.

It is now known that the FBI interviewed the elder Tsarnaev brother in 2011 at the request of the Russian government, which raised concern that he had links to radical Islam. On Sunday rebels in his ethnic homeland of Chechnya denied any link. But a six-month visit to Russia last year is now being scrutinised to see if Tsarnaev did visit any other countries in the North Caucuses region.

Michael McCaul, Republican chair of the House homeland security committee, told CNN's State of the Union on Sunday that the FBI had questions to answer. "He was interviewed by the FBI in 2011 and let go. He travelled back to Russia and spent six months there," McCaul said.

On his return he immediately posted jihadist videos on YouTube, McCaul said. "Clearly something happened in that six-month timeframe. He radicalised at some point." McCaul said he wanted to know why no flags were raised on Tsarnaev that would have helped officials track his movements.

It was reported Sunday that a hold was placed on a citizenship request by the 26-year-old as a result of the FBI's previous interest in him. The New York Times claimed officials at the Department of Homeland Security decided not to grant his application after a routine background check uncovered the 2011 interview by agents.

The FBI has not said publicly who requested that they look into the activities of the elder brother, but it has been assumed that it was the Russian government. In a statement, the bureau admitted interviewing Tamerlan and other family members. "The FBI did not find any terrorism activity, domestic of foreign, and those results were provided to the foreign government in the summer of 2011," it said.

On Sunday, the FBI was accused of having "dropped the ball" regarding the older Tsarnaev brother. "The FBI missed a lot of things is one potential answer, or laws do not allow the FBI to follow up in a sound, solid way," a Republican senator, Lindsey Graham, told CNN's State of the Union.

Peter King, chairman of the House subcommittee on counterterrorism, criticised the FBI. "This is the firth case I'm aware of where the FBI has failed to stop someone," he told Fox News Sunday, going on to cite the cases of al-Qaida recruiter Anwar al-Awlaki, Little Rock shooter Carlos Bledsoe, the accused Fort Hood killer Nidal Malik and alleged American-Pakistani terrorist David Coleman Headley.

"This is the latest in a series of cases like this … where the FBI is given information about someone as being a potential terrorist. They look at them, and then hey don't take action and then they go out and commit murders," King said.