Why does Washington continue to allow hostile Somali Muslims to migrate to the United States?

The FBI is investigating reports that a suicide bomber who killed two African Union soldiers in Mogadishu, Somalia, was an American citizen. According to the African Union, several Islamic extremist insurgents had disguised themselves as Somali government forces in order to attack an African Union post. Days after the suicide bombing, al-Shabaab, a Somali militant group allied with al Qaeda, posted a statement on its website identifying the suicide bomber responsible for the bombing as Abdullahi Ahmed, a Somali-American from Minneapolis. In 2008, a sizable group of young Somali-American men left Minneapolis - home to America’s largest Somali population - and were feared recruited by al-Shabaab. Al-Shabaab has been waging a war against Somalia’s government in an effort to implement a stricter form of Islamic law, or sharia law. Somalia has not had a stable government since 1991, and fighting between the rebels and government troops has added to the impoverished East African nation’s humanitarian crisis.