U.S. counterterrorism officials have not reported such high numbers of Americans joining the Somali terror organization, al-Shabab. The government has said at least 21 Somali-Americans are believed to have traveled to Somalia to join the terror group in what began as a push to expel Ethiopian soldiers. Al-Shabab claimed responsibility for the double suicide bomb attack in Uganda's capital last year, and members have aligned themselves with other anti-Western terror groups.
New York Republican Rep. Peter King's findings are based on his committee's investigation into the threat.
King plans to address the problem Wednesday during his third congressional hearing on Muslim radicalization.