With one suspect dead and the other lying seriously wounded in a hospital, the FBI investigators are reviewing the suspected Boston Marathon bombers' motives and a visit made by one of them to Muslim-dominated Chechnya and Dagestan republics in the Russia's north Caucasus region.
26-year-old Tamerlan Tsarnaev, who was killed in clashes with police in Watertown on Friday, traveled to Russia from Boston several times in recent years, according to US officials who have reviewed his passport file.
He spent six months in Dagestan in 2012 and analysts said that sojourn might have marked a crucial step in his alleged path toward the bombings, according to the New York Times.
Both the suspects were brothers from Chechnya, which has witnessed deadly bombings carried out by Islamic rebels.
The FBI in 2011 interviewed Tsarnaev, suspected of being behind the deadly Boston blasts that claimed three lives and injured over 180, at the request of a foreign government (now acknowledged by officials to be Russia) that suspected he might have ties to extremist groups with links to al-Qaeda.
The senior law enforcement official said the Russians feared he could be a risk, and "they had something on him and were concerned about him, and him traveling to their region."
Chechen extremists pose a greater threat to Russia than they do to the US, counter-terrorism specialists say, though some of the groups have had ties to al-Qaeda, the paper said.
The US investigative agents apparently let him out of their sights after not detecting any terrorist activity.
"The request (by the foreign government) stated that it was based on information that he was a follower of radical Islam and a strong believer," the FBI said.
Agents also interviewed Tsarnaev's family members, the FBI said, but did not detect terrorist activity.
"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," it said in a statement.
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