The FBI this month announced a Minnesota man admitted to having a role in a plan to recruit Somali men to travel to the country and fight against Ethiopian forces.
Al-Shabaab is attempting to create an Islamic state in Somalia, which hasn't had a functioning government since the 1990s. The al-Qaida-affiliated group has declared war on the peacekeeping force in the war-torn country.
A report from the House Committee on Homeland Security concluded there was a "looming danger" of U.S. nationals who have pledged loyalty to al-Shabaab returning to the United States to strike or help al-Qaida and its affiliates attack the United States.
U.S. Rep. Peter King, R-N.Y., chairman of the House committee, said U.S. intelligence agencies have underestimated the threat posed by the Taliban and al-Qaida in the Arabian Peninsula and can't afford the same mistake with al-Shabaab.
King's committee found that federal indictments tied to al-Shabaab account for the largest number of domestic terrorism cases filed by the U.S. Justice Department during the last two years.
In its report, King's committee found that at least 15 U.S. citizens and three Canadians were killed fighting alongside al-Shabaab. The al-Qaida group has the capability to conduct attacks on the United States, the committee found.
"With al-Shabaab's large cadre of American jihadis and unquestionable ties to al-Qaida, particularly its alliance with AQAP, we must face the reality that al-Shabaab is a growing threat to our homeland," King said in a statement.