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FBI agents are working closely with Russian security officials to reconstruct Tamerlan Tsarnaev’s activities and connections in Dagestan during his six-month visit last year, tracking meetings he may have had with specific extremists, his visits to a radical mosque and any indoctrination or training he may have received, law-enforcement officials said Sunday.

At the same time, the bureau is also still looking for “persons of interest” in the United States who may have played a role in the radicalization of Tsarnaev, 26, and his younger brother Dzhokhar, 19, before the Boston Marathon bombing April 15, Rep. Mike Rogers, R-Mich., chairman of the House Intelligence Committee, said Sunday. But Rogers, speaking on the ABC program “This Week,” said “the big unknown” remains what happened in Russia.

Investigators think it is likely the brothers were self-radicalized and got their bomb-making instructions from the Internet. But they are still exploring whether others in Russia or the United States were key influences, if not accomplices, and officials say it may be weeks before the full picture is clear.

Officials said they were still examining the conduct of the brothers’ mother, Zubeidat Tsarnaeva, and Tamerlan’s wife, Katherine, who converted to Islam when she married him in 2010.

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On Saturday, the Russian investigative newspaper Novaya Gazeta reported Tamerlan had sought to join the Muslim insurgency in Dagestan and had been in contact with several rebels who were killed by Russian authorities in 2012 while he was staying in Makhachkala, the regional capital.

Tamerlan Tsarnaev left Dagestan in July 2012, just two days after a shootout between extremists and the police in which several rebels were killed, including William Plotnikov, 23, a Russian-born Canadian, and like Tsarnaev, an amateur boxer. Investigators are trying to determine whether Tsarnaev and Plotnikov met, one official said Sunday.

In 2011, Russian officials sent a warning about Tamerlan Tsarnaev’s extremist views to both the FBI and the CIA, saying they thought he was coming to Dagestan, a republic in southern Russia, to connect with underground groups. That warning was based on telephone conversations intercepted by Russian intelligence, including one between Tamerlan and his mother, in which they discussed jihad, Russian authorities have told the FBI.

Experts on the effort by Russian authorities to contain the Muslim insurgency in Dagestan said that if officials were aware of Tsarnaev’s arrival in January 2012, he probably would have been under scrutiny throughout his time there.

It is unclear how closely the police were tracking Tsarnaev, but his mother described at least one instance in which her son was stopped along the beach in Makhachkala, where Tsarnaev’s parents live, and brought in for questioning.

Videos posted by Tamerlan Tsarnaev indicate he was familiar with Muslim rebel leaders in Dagestan, and investigators have been seeking to determine if he met with any of them.

The account in Novaya Gazeta said one of Tsarnaev’s contacts was Mahmoud Mansur Nidal, who was killed May 19 after a standoff with Russian authorities at an apartment house in Makhachkala. Nidal took several hostages, according to the news agency Interfax, and at one point threw a grenade at the authorities. The hostages were released, but Nidal refused to surrender and was shot dead, Interfax reported.

Another possible contact was Plotnikov. He had been trained in boxing by a well-known Russian coach in Canada and was known among the Muslim rebels in Dagestan as “The Canadian.”

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