US lawmakers learned on Tuesday that security monitors "pinged" when a Boston bombing suspect flew to Russia in 2012, as concerns grew about possible intelligence sharing failures.
Investigators are probing the six-month trip made by Tamerlan Tsarnaev to the troubled regions of Dagestan and Chechnya - home to fierce Islamist and separatist groups - and whether he was radicalized or trained there.
US lawmakers have questioned why authorities were not keeping a closer eye on the 26-year-old - killed in a shootout with police last week - even after he was interrogated by the FBI in 2011 at Russia's request.
Asked by a senator whether US officials had been alerted to Tsarnaev's travel abroad, Homeland Security secretary Janet Napolitano acknowledged that "the system pinged when he was leaving the United States" for Russia in 2012.
But she added that "by the time he returned, all investigations in the matter had been closed."
A year earlier, Tsarnaev had been questioned by the Federal Bureau of Investigation at the request of Moscow, but investigators apparently concluded he was not a threat.
"We will learn lessons from this attack, just as we have from past instances of terrorism," Napolitano said, speaking at a Senate Judiciary Committee hearing on immigration.
Members of Congress were meanwhile given classified FBI briefings about the ongoing investigation into Tamerlan Tsarnaev and his younger brother Dzhokhar, 19, who is hospitalized and in custody on terror charges.
The two ethnic Chechen brothers, who had been living in the United States for over a decade, are accused of the twin marathon bombing on April 15, which killed three people and wounded more than 260 others.
Republican Senator Lindsey Graham had said Monday that an FBI official told him Tsarnaev's trip was not flagged to the agency because his name was misspelled in the system, possibly on a plane ticket.
Graham pressed Napolitano on the issue Tuesday.
She acknowledged "there was a mismatch there," adding that an immigration reform bill now under debate would cut down on such problems by requiring passports to be digitally readable.
It was not immediately clear whether Tsarnaev's departure set off a government alert because he was on a terror watchlist or on a broader, central repository of some 500,000 names, known as the TIDE database.
"How could the Homeland Security Department know this guy was leaving the country and the FBI not?" a frustrated Graham told reporters. "Somebody needs to fix that."
Senate Republican Susan Collins, who attended a briefing with other Intelligence Committee members, expressed frustration at the lack of sharing of "critical investigative information" among agencies.
"That is troubling to me, that this many years after the attacks on our country in 2001, that we still seem to have stovepipes that prevent information from being shared effectively - not only among agencies but also within the same agency in one case," she said, referring to the September 11 attacks.
Saxby Chambliss, the committee's top Republican, said it appeared there had been some "stonewalls and some stovepipes" built up around some agencies, but that they seemed unintentional.
"I don't see anybody yet who dropped the ball, (but) that may develop," Chambliss told reporters after the briefing by FBI deputy director Sean Joyce.
The committee's Democratic chair Dianne Feinstein warned of the "long, arduous task" of piecing together the attack and what may have motivated the suspects, but insisted intelligence agencies were working well together.
"I have never in my lifetime seen an effort as positive as this one is in terms of the cooperation and communication between law enforcement and investigative agencies, and I believe they will get to the bottom of it eventually," she said.