A senior congressional aide privy to the Boston Marathon terror investigation confirmed Saturday that the FBI received the warning after Tsarnaev’s apparently suspicious activities caught the attention of Russian authorities keeping close surveillance on militant Islamist groups in the Caucasus region of the former Soviet Union.

The FBI acknowledged Friday that it had investigated Tsarnaev in 2011, even interviewing him and his family, but “did not find any terrorism activity,” either domestic or foreign.

“The FBI had this guy on the radar and somehow he fell off,” said the congressional aide, who said oversight committees on Capitol Hill are seeking answers from counterterrorism officials. “We heard for several days leading up to this there was no intelligence. Now we know there could have been intelligence.”

Tsarnaev, 26, who was killed in a firefight with police in Watertown early Friday morning, had traveled from Boston to Russia several times in recent years, including an extended stay in 2012, according to multiple US officials who have reviewed his passport file.

Tsarnaev and his younger brother, Dzhokhar, are ethnic Chechens who came to the United States from Kyrgyzstan, a former Soviet Republic in central Asia.

The bureau declined to answer questions Saturday about whether it revisited its 2011 investigation of Tamerlan Tsarnaev after the Marathon attack, or why the bureau was unable to identify the suspects in race day security footage two years after interviewing him and his family.

Dzhokhar Tsarnaev, 19, was captured by police Friday night in Watertown after eluding a manhunt for more than 20 hours. He is in Beth Israel Deaconess Medical Center in “serious but stable” condition and not yet able to communicate, Governor Deval Patrick told reporters Saturday.

As questions mounted over the FBI’s prior experience with the alleged bomber, Greater Boston reveled in the capture of the alleged terrorist and took another step to returning to normal.

Friday’s Boston Red Sox game was canceled while Dzhokhar Tsarnaev was on the loose, but Saturday’s game was held as scheduled, the team’s first home game since last Monday’s bombing.

Members of law enforcement who pursued the alleged bombers were honored on the Fenway infield before the game. Fans cheered Boston Police Commissioner Edward Davis as if he had just shut out the Yankees.

Red Sox designated hitter David Ortiz, in the lineup for the first time this season after an injury, took the microphone and shouted: “This is our [expletive] city, and nobody is going to dictate our freedom. Stay strong.”

While the return of the Red Sox brought some sense of normalcy to the city, 52 people hurt in the bombing remained in Boston hospitals on Saturday, and the heart of Back Bay, where the bombings took place, remained closed for the continuing investigation.

The dramatic encounter that led to the death of one of the brothers and the capture of the other followed a decision Thursday by the FBI to release video and photographs of the suspects, in an effort to have the public help identify them.

The images showed Tamerlan Tsarnaev in a dark cap and carrying a backpack — believed to have held a bomb — on Boylston Street.

Dzhokhar Tsarnaev is pictured in a white cap, also carrying a backpack.

The decision to release the photos came after two days of silence from the FBI, during which they tracked down thousands of tips without making an arrest.

But hours after the images were released, the suspects allegedly unleashed a wave of shocking violence, fatally shooting Massachusetts Institute of Technology police officer Sean Collier in Cambridge, then carjacking a motorist and engaging in a gun battle with Watertown police in a residential neighborhood. MBTA transit officer Richard H. Donohue Jr. was shot in the groin and hospitalized.

Davis conceded in a Globe interview Saturday that “releasing the photos may have led to the further attack” against Collier, which Davis has called an assassination. But he added: “We don’t know that.”