Somali-American students raised in Minneapolis are giving up dreams of becoming doctors or entrepreneurs to instead join a militant Islamist group aligned with Al Qaeda that is fighting to overthrow the shaky Somali government. More than 20 students are the focus of what may be the most significant domestic terrorism investigation since Sept. 11. Members of the jihadist group are said to have carried out beheadings, amuptations, and the stoning of a 13-year-old rape victim. One of the men, Shirwa Ahmed, blew himself up in Somalia in October, becoming the first known American suicide bomber. The men—who, growing up embraced, basketball and the prom, and the Mall of America—appear to be motivated by a mix of politics, faith, and a feeling of loyalty to their homeland, and they are trying to recruit other young Americans to their cause. “This case is unlike anything we have encountered,” said Ralph S. Boelter, the F.B.I. agent leading the investigation.