WASHINGTON — For months, as one young person after another in Minneapolis’s Somali community tried to join the Islamic State terrorist group, rumors circulated of a sinister terrorist recruiter who must be luring gullible teenagers and providing the cash to buy air tickets to Syria.

But 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.

In other words, said Andrew M. Luger, the United States attorney for Minnesota, the circle of friends “recruited each other.” He said they scrounged the money for tickets, selling a car and emptying a financial aid account, and brainstormed about how to evade the F.B.I. and reach the brutal terrorists they idolized.

The F.B.I. is increasingly concerned about this model of radicalization by peers. Because discussions of the Islamic State took place during pickup basketball games and visits to the mall, the wave of recruitment was difficult for the authorities to detect in advance. It is also a source of distress to parents in the Somali-American community, local activists say, providing no nearby villains to blame for leading their children astray.