Oct. 29 (UPI) -- Human rights organization Amnesty International said Wednesday that a U.S. airstrike that hit a Houthi detention center in Yemen in April should be investigated as a war crime. The April attack on Saada, in the northwestern part of Yemen, was part of Operation Rough Rider and killed civilian migrants held in a Houthi detention center because of their immigration status, Amnesty said . The migrants often come through Yemen from the horn of Africa to get to Saudi Arabia for work.