The U.S. announced charges against eight men this week, and agents allege three of them helped persuade men to join an extremist group.
Ralph Boelter, special agent in charge of the Federal Bureau of Investigation's Minneapolis office, said the notion of domestic radicalization "is a moving target. It's evolving. These young men traveled without consulting their families. Now it's happened; now we should all be on guard."
Patrick Rowan, who was chief of the Justice Department's national-security division when the Somali probe began, said: "If there was ever a subconscious belief in the counterterrorism community that our society is different from Europe and therefore we don't have to worry about problems like they've had, the activities in Minneapolis demonstrate we can't think that way."
The cases come amid a flurry of terror investigations in which radicalization may have played a role. In September, federal prosecutors charged an Afghan national in what authorities believe was the most advanced al Qaeda plot to bomb targets in the U.S. since the Sept. 11, 2001, attacks. Authorities are also investigating whether radicalization was involved in the case of Maj. Nidal Malik Hasan, the Army psychiatrist charged with killing 13 people at Fort Hood, Texas.
The Somali cases are different because the motivation of the Somalis was to fight for their homeland. Still, investigators fear that once indoctrinated abroad, such youths could turn their attention to the U.S.