WASHINGTON (CNN) -- Boston bombing suspect Tamerlan Tsarnaev was not on a terrorism watch list or a "no-fly" list when he traveled to Russia last year, a federal law enforcement official told CNN on Tuesday.
The source, who spoke on condition of not being identified, noted the FBI found no suspicions of terrorist ties when it interviewed Tsarnaev and his family members and friends in 2011 after Russia asked U.S. authorities for information on the immigrant from the Caucasus region.
Because the United States "never deemed him a threat," Tsarnaev "was not on a terror watch list or any 'no-fly' list," according to the official.
The issue has raised questions about FBI screening of Tsarnaev, 26, the older of two brothers accused of setting off two bombs that killed three people and injured more than 260 near the finish line of last week's Boston Marathon.
Tsarnaev and his brother, Dzhokhar, also allegedly killed a university policeman on Thursday, three days after the bombings, to set off an unprecedented manhunt in Boston and some suburbs.
Tamerlan died after a shootout with police on Thursday night, and Dzhokhar was captured on Friday.
Now members of Congress are asking how someone the FBI questioned two years earlier because Russia was concerned about his shift toward Islamic extremism could have avoided closer scrutiny since then.
FBI officials began briefing congressional committees later Tuesday to answer questions raised by legislators of both parties.
The closed briefings were expected to focus on Tamerlan Tsarnaev's six-month trip to Russia in 2012 that family members said included visits to Chechnya and Dagestan, regions known for radical Islamic insurgency.
At a hearing earlier Tuesday by the Senate Judiciary Committee, some GOP panel members asked Homeland Security Secretary Janet Napolitano about the Tsarnaev case.
Republican Sen. Chuck Grassley of Iowa cited reports that U.S. authorities were unaware Tsarnaev had left the country for Russia in January 2012 even though his name was misspelled on his airline ticket by Russian carrier Aeroflot, which would normally trigger increased scrutiny.
Napolitano responded that Tsarnaev's departure did "ping" in the homeland security screening system, but she noted that because the FBI's investigation in 2011 found no suspicious activity, there was no reason to follow up.
"There was a missed match there" involving the incorrect spelling of Tsarnaev's name, Napolitano said, adding that "even with the misspelling, in our current system there are redundancies and so the system did ping when he was leaving the United States."
The federal law enforcement official told CNN that such a hit in the system doesn't prompt automatic action. In Tsarnaev's case, the Russian government knew he would be traveling to Russia and had family there, so there was nothing to follow up, the official said.
CNN law enforcement analyst Tom Fuentes, a former FBI official, offered a similar explanation Tuesday.
"By the time he comes back, the FBI case is closed and, again, no additional information comes back from the Russians to keep an eye on him or that he's on his way back to your country," Fuentes said. "Once the FBI case is closed, there is no further monitoring by the FBI of his activity or whether he's going to these Jihadi Web sites or becoming increasingly radicalized."
Tsarnaev was an immigrant from the volatile Caucasus region of southwest Russia who had legal residence in the United States and sought last year to become fully naturalized, like his brother Dzhokhar, 19.
However, the Department of Homeland Security rejected the citizenship request due to the FBI questioning before the Russia trip.
An FBI statement Friday said a foreign government -- later identified by legislators as Russia -- asked for information on Tsarnaev "based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups."
In response, the FBI said, it "checked U.S. government databases and other information to look for such things as derogatory telephone communications, possible use of online sites associated with the promotion of radical activity, associations with other persons of interest, travel history and plans, and education history."
"The FBI also interviewed Tamerlan Tsarnaev and family members," said the FBI statement. "The FBI did not find any terrorism activity, domestic or foreign, and those results were provided to the foreign government in the summer of 2011."
In addition, the FBI "requested but did not receive more specific or additional information from the foreign government," its statement said.
That failed to satisfy Feinstein, who told CNN on Monday that she hoped Tuesday's briefing by FBI intelligence officials would provide information on what Tsarnaev did during the trip.
"And when he came back to this country, why didn't it ring a bell with the FBI intelligence unit that he should be checked out and vetted again?" she said.
Feinstein also noted that Homeland Security officials later denied Tsarnaev's application for citizenship, raising another question about who knew what about him.
The purpose of the hearing was "not to criticize, because I am a big fan of the FBI's, but to go back and see that we plug loopholes," Feinstein said.
The lengthy travel to Russia by Tsarnaev, who's ethnically Chechen but came to the United States from Kyrgyzstan, caused some legislators and analysts to speculate he may have received training during the trip.
Conservative Sen. Lindsey Graham, who said Sunday the FBI may have dropped the ball in its investigation of Tsarnaev, began easing off that claim on Monday.
The South Carolina Republican confirmed he talked to the assistant director of the FBI and learned how the bureau interviewed Tsarnaev, his parents and classmates in 2011.
"They put his name through the system and they sent back this information to the Russians and said, 'Do you have anything else?' And they never got a reply back," Graham said.
Graham also noted that Tsarnaev wasn't flagged upon returning from Russia because of the misspelling of his name on the ticket.
"Now whether or not he intentionally changed his name or Aeroflot just got the spelling wrong, I don't know. That's to be determined," Graham said.
As for apparent warning signs that occurred within the last year, such as YouTube postings of radical Islamists, Graham said the FBI told him "they have limitations on what they can do."
"So maybe it's the system failed, didn't provide the FBI with the tools, or maybe they didn't use it properly," he added. "That's why maybe we need to find out what happened."
His comments sounded similar to those made by House Intelligence Committee Chairman Mike Rogers, who defended the FBI on Sunday.
Rogers told NBC the agency "did their due diligence" but Russian authorities "stopped cooperating" when the United States sought further clarification. Rogers also said he believed Tsarnaev may have traveled overseas using an alias.
CNN law enforcement analyst Tom Fuentes, a former FBI official, said there was little the bureau could do once Russia failed to respond to its request for further details.
"If they don't give you more, then everything that can be done has been done unless you know that there should be more to the story," Fuentes said.
He detailed how the FBI employs what amounts to "triage" to deal with what he said were tens of thousands of similar inquiries a year that require some level of bureau investigation.
"If you are getting this from a hot place like Afghanistan or the tribal area of Pakistan or places where we have had specific training camps and people deployed on purpose to come and attack us, then that is the highest priority," he said. "And even there, many of the people that go back and forth are visiting family. I mean, they are not always going back to be trained to be terrorists or always going back for refresher courses on terrorism."
Regarding Russia, Fuentes noted the ongoing conflict with Chechen separatists that may have caused Moscow's request for information on Tsarnaev.
"That's been an ongoing fight, but it's been localized," he said, adding that he couldn't recall a case in which a Chechen trained at home came to attack the United States.
However, Fuentes noted that al Qaeda had sent people to the Caucasus region for training that included bomb building.
Now U.S. investigators need to find out if the Tsarnaevs "had connections, were they deployed by a bigger group, and are there other terrorists in the United States," Fuentes said.
"Are there other explosive devices hidden somewhere or booby traps created, a cache of weapons?" he wondered. "That'll be the task."
CNN's Jim Acosta, Ted Barrett and Ashley Killough contributed to this report.