Those charged in indictments announced Thursday aren't accused of threatening attacks against the U.S. Most allegedly aided al-Shabaab in recruiting fighters to join its insurgency.

But the indictments point to a broader trend of U.S. citizens and residents joining extremist groups and raise fears among authorities that young men trained and radicalized in Somalia could return to the U.S. and pose a threat here.

"As demonstrated by the charges unsealed today, we are seeing an increasing number of individuals—including U.S. citizens—who have become captivated by extremist ideology and have taken steps to carry out terrorist objectives, either at home or abroad," Attorney General Eric Holder said at a news conference. "It's a disturbing trend that we have been intensely investigating in recent years and will continue to investigate and root out. But we must also work to prevent this type of radicalization from ever taking hold."

The charges were filed in California, Alabama and Minnesota, where the Federal Bureau of Investigation in recent years has tracked the recruitment of young Somalis drawn to train and fight with al-Shabaab. Most were motivated to join the fight in Somalia after Ethiopia, and later the African Union, sent troops to defend Somalia's nominal government from insurgent attack, investigators say.

Federal prosecutors allege that two of the accused, Somali-born women, held fund-raising conference calls with al-Shabaab leaders and went door-to-door to collect funds for the group in Somali communities in the U.S. and Canada. They raised money under the guise of trying to help the needy, prosecutors allege.

Key among those charged is Omar Hammami, a 26-year-old American formerly of Daphne, Ala., who has now appeared in recruiting videos and is now believed to be a major "operational" leader of al-Shabaab, Mr. Holder said. Mr. Hammami, now known as Abu Mansour al-Amriki, or "the American," was charged in Mobile, Alabama. He and most of those charged remain at large and are believed to be in Somalia.

The threat posed by al-Shabaab has grown since last month when the group claimed responsibility for bombings in Uganda that killed dozens of people as they watched the televised World Cup soccer final. Uganda is one of several countries that has supplied troops to the African Union contingent in Somalia.

The new charges announced Thursday are aimed at what investigators say were top U.S. recruiters for al-Shabaab.

The case arose from a probe that began in 2008 when multiple families in an ethnic Somali enclave in Minneapolis reported that their young sons had suddenly disappeared and turned up weeks later in Somalia.

One of those recruits became the first known American suicide bomber in an al-Shabaab attack in Somalia that year, according to U.S. law enforcement officials.

Prosecutors previously charged several alleged recruits, mostly from Minnesota, some of whom underwent training at Al-Shabaab camps and returned to the U.S.