Boston bombers radicalized on U.S. soil?

Updated at 12:01 p.m. Eastern

LONDON A federal law enforcement source tells CBS News investigators are working to determine whether the suspects in the Boston Marathon bombings, brothers Tamerlan, 26, and Dzhokhar Tsarnaev, 19, have links to other terrorist cells, foreign or domestic.

Officials are trying to determine whether the brothers, who lived for at least a short period near Russia's restive Chechnya region, were acting independently or directed by a broader conspiracy. The source told CBS News producer Pat Milton that investigators and the intelligence community are pressing to determine if the two men were directed by a central player or someone overseas.

According to a former teacher at a school in the Russian region of Dagestan, the brothers were two of four siblings who came to the area with their parents as refugees from Kyrgyzstan in 2001. They lived only about one year in Dagestan, a region next door to Chechnya. Both regions have a long history of Islamic extremism and a number of well-established terrorist groups.

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Ethnic Chechens live across the region, and it is still believed that the Tsarnaevs belong to that ethnic group.

Dzhokhar, born in 1994 as a war began between Chechen separatists and the Russian government, had most recently viewed videos on YouTube depicting atrocities from the front lines of Syria's civil war and of Chechen separatist propaganda.

There has been no indication that the Tsarnaev brothers are linked, however, to Chechen terror groups operating in the region now. The former head of the school who spoke to the Reuters news agency in Dagestan said the family left for the U.S. in 2002.

Chechnya's leader, Ramzan Kadyrov, dismissed any links between the brothers and Chechnya.

"Any attempt to draw a connection between Chechnya and Tsarnaevs -- if they are guilty -- is futile. They were raised in the United States, and their attitudes and beliefs were formed there," he said in a statement Friday.

Even America's top diplomat, Secretary of State John Kerry, cautioned against drawing links to the troubled region, saying Friday that, "it would be inappropriate," to talk about any Chechnya connection to the bombing suspects.

Neil Doyle, a British author and expert in online extremism and radicalization, told CBSNews.com on Friday that the young man's recent viewing "would suggest perhaps a large degree of self radicalization, but we can't rule out the possibility that there may have been someone local to them, an imam or someone, who influenced the process."

Charles King, a professor at Georgetown University and an expert in the Caucus region, where Chechnya is located, says links to Chechen radicals in the U.S. are also unlikely, as there are simply too few of them.

He describes the ethnic Chechen community in the U.S. as "very, very small," and scattered across the country. "I really think the American angle on this story is much more powerful than the Chechen one," adds King. "It's a story of how young people get radicalized."

As Doyle points out, there is no shortage of jihadist propaganda available on the Internet, courtesy of al Qaeda and its various branches and like-minded organizations.

The bombs used in the Boston attack were made using pressure cookers -- not a new practice, by any means, but one which was laid out in a detailed how-to manual published online by al Qaeda's Yemen branch in the last two years.

Chechnya and Dagestan, predominantly Muslim regions of Russia, have seen decades of war and internal violence as separatists vie for independence from Moscow and for internal power.

The most prominent Islamic terror group in Chechnya is called the Islamic Emirates of the Caucuses, led by Doku Umarov.

While the group is not believed to have direct, operational cooperation with al Qaeda's central leadership, it shares the same jihadist ideology. In 2011, the United Nations Security Council added Umarov to a list of people officially linked to al Qaeda.

Umarov's group was behind the March 2010 Moscow metro bombing carried out by two female suicide bombers which left 40 people dead. It was also blamed for a 2011 attack on the Domodedovo airport, near Moscow, which took 36 lives.

More recently, Chechen jihadists have joined various rebel militias fighting against President Bashar Assad in Syria's two-year civil war.

Thus far, there have been no terrorist attacks on the U.S. or other Western nations planned or carried out by Chechen groups -- their focus has been exclusively on Russia.