Homeland Security Secretary Janet Napolitano said Tuesday the US government's security system "pinged" when one of the Boston bombing suspects flew to Russia last year.

Her comments at a Senate Judiciary Committee hearing came as lawmakers have questioned why US authorities were not keeping a closer eye on Tamerlan Tsarnaev, 26, even after he was questioned by the FBI in 2011.

The confusion over what US officials knew about the six-month trip by Tsarnaev -- who was killed last week in a shootout with police days after the deadly blasts at the Boston Marathon -- was expected to be raised at two closed-door intelligence briefings Tuesday in the Senate and House.

Napolitano told an earlier hearing on immigration 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."

Investigators are probing the six-month trip made by Tamerlan to Russia's troubled regions of Dagestan and Chechnya -- home to fierce Islamist and separatist groups -- and whether he was radicalized or trained there.

A year earlier, Tsarnaev had been questioned by the FBI 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.

Republican Senator Lindsey Graham had said Monday that a senior official at the Federal Bureau of Investigation told him US authorities missed Tsarnaev's trip because his name was misspelled in the system, possibly on a plane ticket.

Graham pressed Napolitano on the issue Tuesday.

Napolitano acknowledged "there was a mismatch there," adding that an immigration reform bill now under consideration would cut down on such problems by requiring passports to be readable electronically.

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, maintained by the National Counterterrorism Center.

"What I was told two days ago is not holding up to scrutiny," a frustrated Graham told reporters after the immigration hearing.

"How could the Homeland Security Department know this guy was leaving the country and the FBI not? Somebody needs to fix that."

Republican Senator John McCain said he is calling for congressional hearings to determine whether US authorities missed warning signs about the suspect.

"Apparently there's two different stories here. That needs to be reconciled, that's why we'll have hearings," McCain said.

Tamerlan's 19-year-old brother and alleged accomplice Dzhokhar was captured Friday and is being held on terror charges.