WASHINGTON -- The Justice Department announced Thursday that it has filed terrorism charges against 14 people accused of supporting the Somali extremist group al-Shabab and said two of the suspects had been arrested in Minnesota.

The indictments stem from an ongoing federal investigation into what authorities called "a deadly pipeline" between U.S. cities and the increasingly brutal Somali militia, which is thought to have carried out last month's bombings in the East African nation of Uganda that killed more than 70 people.

Twelve suspects, including at least six U.S. citizens, were accused of leaving the United States to fight alongside al-Shabab. FBI officials say the men -- hailing from Minnesota, San Diego and Daphne, Ala. -- are in Somalia.

In announcing the charges, Attorney General Eric Holder said authorities were discovering "a disturbing trend" of Muslim Americans being recruited to fight with or support terrorist groups. In Minnesota, which is home to some 50,000 ethnic Somalis, authorities have charged a total of 19 defendants in connection with the investigation into al-Shabab.

"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," Holder said.

An annual State Department report on global terrorism released Thursday called al-Shabab the "second deadliest group," saying it was responsible for more attacks last year than any group except the Taliban in Afghanistan. The militia is one of al-Qaida's most active affiliates, the report said, and has grown rapidly in size and ambition since 2007, when it began battling a weak interim government for control of the Somali capital, Mogadishu.

Authorities identified the individuals arrested as Amina Farah Ali, 33, and Hawo Mohamed Hassan, 63, naturalized U.S. citizens living in Rochester, Minn. The women were charged with conspiring to raise money for al-Shabab through the Islamic money-transfer system known as hawala.

In separate indictments, the Justice Department added six names to the list of U.S. citizens who have allegedly traveled to Somalia to join al-Shabab.