On Monday, federal officials, announcing their biggest Islamic State recruitment case to date, said there was, in fact, no recruiting mastermind. Instead, for the six men arrested, there was just the camaraderie of sharing an illicit ideology, plus advice and inspiration by phone and Internet from one of their friends, a young Minneapolis man who joined the Islamic State last year. All of those charged were Somali-Americans ages 19 to 21 from Minneapolis, where four were arrested on Sunday.

The F.B.I. is increasingly concerned about this model of radicalization by peers. The number of Islamic State recruits from the United States remains small in comparison with Western Europe, where more than 3,000 people are believed to have traveled to Syria to join the group. But law enforcement and intelligence officials have tried to disrupt the travel in part because they were concerned that Americans could train with the Islamic State and then return to the United States to carry out attacks.

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