“They had something on him and were concerned about him and him traveling to their region,” the official said. The F.B.I. conducted a review, examining Web sites that he had visited, trying to determine whether he was spending time with extremists and ultimately interviewing him. The F.B.I. concluded that he was not a threat. “We didn’t find anything on him that was derogatory,” the official said. The F.B.I. released a statement late Friday confirming it had scrutinized Mr. Tsarnaev but “did not find any terrorism activity, domestic or foreign.” It had requested more information from the foreign government, it said, but had not received it.

Now officials are scrutinizing that trip, to see if he might have met with extremists while abroad.

The brothers were born in Kyrgyzstan, an official said, and were of Chechen heritage. Chechnya, a long-disputed Muslim territory in southern Russia, sought independence after the collapse of the Soviet Union and then fought two bloody wars with the authorities in Moscow. Russian assaults on Chechnya were brutal, killing tens of thousands of civilians as terrorist groups from the region staged attacks in central Russia.

The older brother, Tamerlan Tsarnaev, traveled to Russia from the United States early last year and returned six months later, on July 17, a law enforcement official said. His father, Anzor Tsarnaev, said his son had mostly stayed with him at his home in Makhachkala, the capital of the Dagestan region, but that the two men had also visited Chechnya.

“We went to Chechnya to visit relatives,” Mr. Tsarnaev said in an interview in Russia.

The trip will come under intense scrutiny to determine whether he met with extremist groups or received training, current and former intelligence and law enforcement officials said. Kevin R. Brock, a former senior F.B.I. and counterterrorism official, said, “It’s a key thread for investigators and the intelligence community to pull on.”

Anzor Tsarnaev, who maintained that his sons were innocent and had been framed, said that during the trip to Chechnya his son had “only communicated with me and his cousins.”

Image The Federal Bureau of Investigation released this image of Dzhokhar A. Tsarnaev. Credit... Reuters

The hunt for the bombing suspects took a violent turn Thursday night when two men believed to be the Tsarnaev brothers fatally shot an M.I.T. police officer, Sean A. Collier, 27, in his patrol car, the Middlesex County district attorney’s office said. Soon after, two armed men believed to be the brothers s carjacked a nearby Mercedes S.U.V. and drove off with the driver in the car.