Majority Investigation Report - Al Shabaab: Recruitment and Radicalization Within the Muslim American Community and the Threat to the Homeland, U.S. House of Representatives, Committee on Homeland Security, One Hundred Twelfth Congress, July 27, 2011 [open pdf - 393KB]
"The Majority staff of the Committee on Homeland Security has conducted an investigation into the threat by al-Shabaab al-Mujahideen in Somalia, Al Qaeda's major ally in East Africa, and its efforts to radicalize and recruit Muslim-Americans inside the U.S. The key finding is that there is a looming danger of American Shabaab fighters returning to the U.S. to strike or helping Al Qaeda and its affiliates attack the homeland. U.S. intelligence underestimated the Pakistani Taliban and Al Qaeda in Yemen's capability of launching attacks here; we cannot afford to make the same mistake with Shabaab."
U.S. House of Representatives, Committee on Homeland Security: http://homeland.house.gov/