In 2005, at the age of twenty-four, Mostafa traveled to Yemen before further traveling to live in Somalia where he joined the Jihadist militant group al Shabaab.[1][2][3] Mostafa swiftly rose up the ranks of the group, serving as an explosives expert, propaganda expert, military instructor, and interpreter for the organization.[8][4][1] He rose to a height in prominence within the organization in 2009 and is currently believed to be the number one most influential United States citizen partnered with an overseas terrorist organization.[3][6][2]