Briefing to senators raises questions about whether information was effectively passed on by FBI to other agenciesCommunication 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.