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As more information emerges on Tamerlan Tsarnaev, the older of two Chechen immigrant brothers accused of carrying out the Boston Marathon bombings, the clearer the picture becomes of a radicalized Muslim whose shallow-rooted ideology was fed by a deepening hatred of US counterterrorist policy in Muslim countries.

Less clear is where that radicalization 鈥� and the germination of the bombing plot 鈥� occurred. Federal investigators want to know if Mr. Tsarnaev, killed early Friday in a shootout with police, was essentially a home-grown terrorist, or if the 26-year-old received any training and direction from foreign extremist groups 鈥� in particular during a six-month trip to his native North Caucasus region of Russia last year.

Russia seems likely to cooperate with US authorities on the Tsarnaev investigation, some regional experts say, especially because the threat of Islamist extremism and terrorism is a concern the Russians have long emphasized in their dealings with the US and on a variety of issues 鈥� from human rights abuses to the war in Syria. Russian President Vladimir Putin sent President Obama his condolences over the Boston bombings even before the Tsarnaev brothers emerged as suspects.

Still, some analysts wonder if a recent row between the two countries over human rights abuses and what Russia interpreted as US interference in Russia鈥檚 domestic affairs suggest that lingering suspicions could hamper cooperation on the bombing case.

The FBI declined to discuss any plans to investigate with Russian authorities Tamerlan鈥檚 2012 trip to Russia or other places he visited. But one US official noted that the FBI has a legal attach茅 in Moscow and one in Kiev, Ukraine, and that one could 鈥渟afely assume鈥� the investigation is already under way. 聽 聽聽

The younger Tsarnaev, Dzhokhar, was formally charged Monday with use of a "weapon of mass destruction," specifically an improvised explosive device, resulting in death.

As the 19-year-old recovers in a Boston hospital from throat and leg wounds he sustained in the same shootout, investigators are combing through seized computers, questioning contacts, and revisiting a closed file the FBI had opened on Tamerlan Tsarnaev after Russia asked the US in 2011 to investigate the ethnic Chechen as an adherent of radical Islam and for links to extremist groups.

The FBI closed the file after questioning Tamerlan and family members but finding no evidence of contacts with terrorist organizations.

But evidence is surfacing of a radicalization that began at least as early as 2009, when the cars-and-clothes-loving Tamerlan informed an uncle he was giving all that up 鈥渢o do God鈥檚 business.鈥�

And what did the community-college dropout do in Chechnya and Dagestan 鈥� the latter being the focal point of a jihadist anti-Russia insurgency 鈥� when he visited the two North Caucasus regions for six months last year? One obvious question US investigators will ask their Russian counterparts is: Given that you were already worried about Tamerlan in 2011, did you keep tabs on him when he returned last year? If so, what did you learn?

鈥淲hat I want to know is, what did the Russians do when [Tamerlan] went back to Russia?鈥� former Bush White House terrorism adviser Richard Clarke queried Sunday on ABC鈥檚 "This Week." 鈥淒id they follow him around? That鈥檚 a question we need an answer to.鈥�

The brothers鈥� father, who now lives in the Dagestan capital of Makhachkala, says his son returned to renew his Russian passport.

But Dagestan has recently become a focal point of Chechen rebel leader Dokka Umarov, who has come to be known as the 鈥�Osama bin Laden of Russia,鈥� as his war with Russia has transitioned in recent years from a nationalist campaign of independence for Chechnya to one more closely associated with radical Islam.

As the Chechen rebellion has shifted to a broader fight for a reunited Muslim region across central and even south Asia, links between radical Chechens, Uzbeks, and other central Asians and their Al Qaeda counterparts in Pakistan鈥檚 North Waziristan region have strengthened, according to some Al Qaeda and Islamist extremism experts.

The Islamic Jihad Union, which the US listed as a Foreign Terrorist Organization in 2005, is based in Waziristan and is known to have recruited Central Asians (as well as Europeans and Arabs) to fight US forces in Afghanistan and to carry out terrorist attacks.

Still, some regional experts say evidence is scant that the leaders of the North Caucasus鈥� Islamist insurgency have an interest in encouraging attacks in the US. As a result, they say, focusing on Tamerlan Tsarnaev鈥檚 2012 Caucasus visit may be a red herring.

In an analysis for IHS Global Insight, country analyst Matt Clements and political risk analyst James Brazier describe the Caucasus insurgency as focused on 鈥渢he essentially local priority鈥� of establishing a Caucasus Emirate.

The insurgency鈥檚 operations are focused on Russian authority, which it considers to be the 鈥渙ccupying鈥� state, the two researchers say. 鈥淭he group has never called for attacks on the US or the West more generally, and if anything has become more locally focused over recent years,鈥� they add.

They also note that Dokka Umarov, considered the mastermind behind attacks such as the 2010 Moscow subway bombings, released a statement in February 2012 declaring that civilians should no longer be targeted.

The IHS researchers find that the Caucasus insurgents would have little reason to encourage attacks on the US. They note, for example, that the US has frequently criticized Russian rights abuses in the region 鈥� a pattern they note continued in this year鈥檚 human rights report issued by the State Department.

鈥淭he more likely scenario is that the brothers were self-radicalized individuals,鈥� the two researchers conclude.

The Russian authorities' claim to the FBI in 2011 that Tamerlan had links to 鈥渦nderground groups鈥� could have been wrong or overzealous. They may have been referring to his postings on Russian social media sites, where he is believed to have linked to jihadist sites, some extolling the establishment of a Central Asian Muslim region.

American investigators will want to know if Tamerlan had any contact with foreign extremists or radical groups, or if he went about his radicalization on his own.

His uncle, Maryland resident Ruslan Tsarni, has no doubts. As he told NBC's "The Today Show" last week, 鈥淭hat so-called radicalization was seeded right here, not in the Caucasus, not in Russia, not in Chechnya, which he had nothing to do with.鈥�