On April 15, law enforcement officials say, Mr. Tsarnaev and his younger brother, Dzhokhar, set off bombs at the Boston Marathon, killing three people and maiming many others.
Should the Russian warnings — seemingly confirmed in part last year when the counterterrorism task force in Boston learned that Mr. Tsarnaev was traveling to Russia — have permitted American officials to foil the marathon plot? That question emerged on Thursday as the crux of a debate among members of Congress, counterterrorism officials and outside experts about whether, and how, the security system failed.
F.B.I. officials have defended their response to the Russian tip, which prompted agents to interview Mr. Tsarnaev and his parents and check government databases and Internet activity. The bureau found nothing alarming and closed the case. The C.I.A. added Mr. Tsarnaev's name to the Terrorist Identities Datamart Environment, a database of people with possible terrorist ties, but took no other action.
On Thursday, some members of Congress and former government officials said Tamerlan Tsarnaev's six-month visit to Dagestan last year was a missed opportunity to refocus attention on him and potentially prevent the attack. Others suggested that the criticism was 20-20 hindsight, and that the F.B.I.'s performance was reasonable under the circumstances.
The critical moment came in January 2012, when a Customs database sent an alert about Mr. Tsarnaev's plan to travel to Russia to a Customs agent assigned to the F.B.I.-led Joint Terrorism Task Force in Boston, according to a Congressional official. It is unclear who else saw the information, but it does not appear to have prompted any new scrutiny of Mr. Tsarnaev at the time or when he returned to the United States that July.
"If there was a failure at any time, maybe it was at that point, to get a follow-up interview," said Representative Mike Rogers, Republican of Michigan and chairman of the House Intelligence Committee, a former F.B.I. agent. "But even so, it's hard to say they did something wrong. Travel in and of itself is not derogatory information, and that area is far down on our priority list."
Across Capitol Hill, senators from both parties emerged from a classified briefing on the bombings sounding generally supportive of the F.B.I. "I wish there would have been more," said Senator Jeff Flake, an Arizona Republican on the Intelligence Committee, "but I'm not in a position to say that I would have done it differently."
Senator Carl Levin, a Michigan Democrat who leads the Armed Services Committee, added, "Unless there's additional information that pops up, I'm not critical of their actions."
But Senator Lindsey Graham, Republican of South Carolina, said in earlier remarks to reporters that the Boston bombing case "is becoming, to me, a case study in system failure."
"You have Russian intelligence services contacting two agencies within our federal government responsible for our national security, the F.B.I. and the C.I.A.," he said. "They tell us, 'We believe you have a radical Islamist in your midst.' " Despite the warning and the F.B.I.'s initial follow-up, Mr. Graham said, Mr. Tsarnaev was able to visit Dagestan and return unnoticed, and discuss "killing Americans" openly on the Internet undetected.
Jimmy Gurulé, a former counterterrorism official who teaches at Notre Dame Law School, said the alert about Mr. Tsarnaev's travel plans should have prompted new attention, since it appeared to give weight to the Russian warning. He said that the authorities should have sought a court warrant to monitor his cellphone and e-mail while he was in Russia. "When he came back to the United States, they should have pulled him out of the Customs line, inspected his belongings, looked at his laptop and cellphone and questioned him about what he had done in Dagestan," said Mr. Gurulé, the former undersecretary of Treasury for terrorism and financial intelligence.
But law enforcement officials said it was unrealistic to expect the F.B.I., which had already taken a hard look at Mr. Tsarnaev, to reopen the case merely because of his travel. The TIDE database has roughly 540,000 names in it, a senior law enforcement official said, and Customs officials get 20 or 30 alerts every day about travel by people in various databases.
In addition, the official said, it would have violated Justice Department guidelines to keep pursuing Mr. Tsarnaev after the initial assessment found no evidence of a crime. "You pursue the original information, come to conclusions," he said.
The official said that the F.B.I. would certainly have looked at Mr. Tsarnaev again if the Russians had told the bureau that they had developed more information on him during his trip. "That is all that would have taken," the official said.
One factor in the failure to follow up may have been Mr. Tsarnaev's ethnicity as a Chechen and his destination, Dagestan, according to both government officials and independent specialists. While those might have set off suspicions in Russia, militants from the Caucasus have generally not targeted the United States.
The authorities would most likely have given Mr. Tsarnaev a closer examination when he returned to the United States if he had traveled to Yemen or Pakistan, where multiple plots against American cities have been hatched. "We can't hassle every person who comes from overseas," the official said. The bureau saw the Tsarnaev matter as "their case, not ours," he said, referring to the Russians. "We have never had a direct threat from there and they are not on the list of priority countries."
