Law enforcement officials are concerned that the recruits, who hold American passports, could be commissioned to return to the United States to carry out attacks here, though so far there is no evidence of such plots.

“The potential implications to national security are significant,” said Ralph S. Boelter, the special agent in charge of the Minneapolis field office of the Federal Bureau of Investigation. He added that the nationwide inquiry would continue with the cooperation of many Somali immigrants and that more arrests might be coming.

The disclosures are the government’s first public account of a recruitment operation that it says has largely focused on Somali-American men from the Minneapolis area. Those young men included Shirwa Ahmed, 26, who carried out a suicide attack in northern Somalia in October 2008, becoming the first known American suicide bomber. Since then, at least five other recruits have been killed in Somalia, relatives and friends say, and four defendants have entered guilty pleas.

The court documents, which included unsealed indictments and criminal complaints, provide chilling details about the experience of the recruits, who began to enlist in Al Shabab in September 2007. They attended training camps in Somalia run by Somali, Arab and Western instructors, who taught them to use machine guns and rocket-propelled grenades and indoctrinated them with anti-American and anti-Israeli beliefs, according to one complaint. Two of the Minneapolis recruits took part in an ambush against Ethiopian troops, and many others were involved in combat, according to the documents.

One of the fighters, Cabdulaahi Ahmed Faarax, later returned to Minneapolis and emerged as a recruiter, officials said. A 32-year-old cab driver and divorced father of two, Mr. Faarax had sustained a leg injury while fighting with Al Shabab, a senior law enforcement official said.