Anders Folk was an assistant U.S. attorney in Minneapolis for several years of the recruiting investigation before leaving for private practice. Al-Shabab's recruiting was at least as effective after the Ethiopians left as before, he said.
"Al Shabab's recruiting technique was essentially a call to jihad, that this is a religious duty," Folk said. "It was a call to jihad to come and fight."
Internet videos are a major tool for the group. Many feature scenes of men with covered faces firing automatic weapons, marching or practicing martial arts, as well as images of dead bodies and religious documents. Some show English-speaking suicide bombers reciting last wills.
The group often appeals to young men who've had trouble assimilating into American life, perhaps because they are unable to get a job, dropped out of school or got involved in gangs, Jamal said.
He cited a recently released al-Shabab propaganda video that lauded three "Minnesotan martyrs," including the American-born non-Somali Troy Kastigar, a convert to Islam.
Smiling and laughing in the footage, Kastigar called his battle experiences "the real Disneyland" and urged other Muslims to come and "take pleasure in this fun." He was killed in 2009 in Mogadishu, according to the video.
The recruiters masquerade "as people who are there for you at your lowest point," said Abdul Mohamed, a spokesman for Ka Joog, a Minneapolis-based nonprofit whose name means "stay away," which works to provide positive alternatives for Somali youth through education, the arts and mentorship.
"Instead of shying away from this issue and letting it separate us, it's best if we take it on headstrong and steadfast so in the future we can prevent it from happening," Mohamed said. "At the end of the day, these kids are full of potential."
Associated Press writer Amy Forliti contributed to this report from New York.