Communication failures between US intelligence agencies may have prevented the detection of the suspected Boston bombers, according to senators briefed in secret by the FBI on Tuesday.

The bureau's deputy director, Sean Joyce, appeared before a closed session of the Senate intelligence committee to address concerns that it did not thoroughly investigate a tip-off from Russian security services in 2011 about Tamerlan Tsarnaev, the older of the two suspects.

While his reassurances appear to have satisfied a number of those present about the initial FBI investigation, they raised fresh questions about whether information was then effectively passed to other agencies such as the Department of Homeland Security, which manages border control.

The homeland security secretary, Janet Napolitano, acknowledged on Tuesday that the return of 26-year-old Tamerlan Tsarnaev into the US in 2012 was not flagged. "The system pinged when he was leaving the United States. By the time he returned all investigations had been closed," Napolitano said at a Senate hearing.

In Moscow, embassy officials said US investigators travelled to southern Russia on Tuesday to speak to the parents of the suspects. The parents of Tamerlan and Dzhokhar Tsarnaev live in Dagestan, a predominantly Muslim province in Russia's Caucasus, where Islamic militants have waged an insurgency against Russian security sources for years.

Investigators want to find out more about the trip that Tamerlan Tsarnaev made in 2012, and what might have motivated the brothers to carry out their attack on the Boston marathon last week, in which three people died and more than 260 were injured. The surviving suspect, 19-year-old Dzhokhar Tsarnaev, was charged at his hospital bed on Monday with using and conspiring to use a weapon of mass destruction, a count that carries a possible death penalty.

After the FBI briefing on Capitol Hill, some senators emerged with concerns about silos, or "stove pipes" in the law enforcement structure. "I am very concerned that there still seem to be serious problems in sharing information," Republican Susan Collins told reporters. "It is troubling to me that many years after 9/11 we still have stove pipes that prevent information being shared internally."

The concerns were echoed by the Republican vice-chairman of the intelligence committee, Saxby Chambliss, who said: "There were a lot of questions on this [from senators]; we need to ask the CIA, FBI, Department of Homeland Security and national counter-terrorism centre for more information."

Democrat senators such as committee chair Dianne Feinstein were more sympathetic. She said she was confident the FBI had not dropped the ball.

The senate intelligence hearing also shed fresh light on the state of the current investigation, which increasingly looks to be focusing on online radicalisation. NBC and the Associated Press reported US officials as saying that preliminary evidence from the interrogation of Dzhokhar Tsarnaev suggested the brothers were motivated by religious extremism but were apparently not involved with Islamic terrorist organisations.

"This is a new type of danger we face," said senator Marco Rubio. "The increasing signs are that these are individuals who were radicalised by Islamic fundamentalist terrorists using internet sources to gather not just their philosophy but also learning components of how to do these things."

The intelligence community has reacted defensively to criticism this week of the Boston case. Bruce Riedel, a former CIA and Pentagon official now at Brookings, said: "You can pretty much count on anything that happens in the US being labelled an intelligence failure within 24 hours." But he said that agencies received so many tipoffs that it was hard to process them all.

Philip Mudd, who was the top intelligence adviser to the FBI leading up to 2011, said: "We are going to see this again, and we are going to ask ourselves: how did we fail? But before you ask that question, how are you going to boil the 10,000 people you interview down to that one case, and how are you going to deal with the 500 false positives?"

Mudd, who also served as deputy director of the CIA's counter-terrorism centre, said the Boston bombings might provoke a reassessment of the dangers of homegrown radicals in the US. "A few years ago we very proudly said we didn't have a problem in the US with radicalised Muslims – that that was a European problem, a British problem."

In Boston, public health officials said the number of people injured in the bombings had risen to 264 after people who suffered minor injuries sought care. The condition of Dzhokhar Tsarnaev was reported to have improved. He was described as being in a "fair" condition at Beth Israel Deaconess Medical Center, the US attorney's office in Boston said on Tuesday. © Guardian News and Media 2013