Al-Shabab is now fighting against an Islamist president
US officials say several cases of US citizens of Somali origin returning to their homeland to join the Islamist al-Shabab militia are being investigated.
A Senate committee heard most of the young recruits came from Minneapolis city in the US state of Minnesota.
Al-Shabab leaders have admitted having links to al-Qaeda but the officials said there was no evidence of Somali-Americans planning to attacks the US.
The US state department considers al-Shabab as a terrorist organisation.
The radical Islamist guerrilla group now controls much of southern and central Somalia.
They came for a better life and this really counters their participation in what is the American Dream and what people think of it
The group continues to fight even though moderate Islamist Sheikh Sharif Sheikh Ahmad has been elected president by parliament.
He has said he will introduce Islamic law, or Sharia.
Andrew Liepman, deputy director for intelligence at the National Counterterrorism Center, said: "They are going to Somalia to fight for their homeland, not to join al-Qaeda's jihad against the United States, so far."He told the Senate Homeland Security committee al-Qaeda did not have strong organisational links to al-Shabab, despite the group's leaders admitting to links.
Somali-American recruits to al-Shabab have included a 27-year-old college student from Minneapolis who blew himself up in Somalia last October.
FBI national security official Philip Mudd said: "Some get there and become cannon-fodder.
"A lot of them are being put on the front line and some of them, I think, have been killed on the front line, from the United States."
Up to 200,000 Somali-Americans live in the US and other population centres for the ethnic group include Seattle, San Diego, Atlanta and Columbus.
Analyst Gregory Pirio, who has written a book on Somalia called The African Jihad, told the BBC's Network Africa programme there was "a lot of distress" in Somali-American communities about these recruits.
"People came here [to America] in large numbers from Somalia in 1991 when Siad Barre's government fell and there was no central government in Somalia and they came for opportunity," he said.
"They came for a better life and this really counters their participation in what is the American Dream and what people think of it."