Arif Uka is a 21-year-old German-Albanian Muslim whose family came from the ethnically divided region of Mitrovica in northern Kosovo. He is being held by German police after the shooting deaths Wednesday of two U.S. Air Force members, and injury to two more—one seriously—in a group headed for Afghanistan via the sprawling Frankfurt International Airport and nearby American military base at Ramstein.

Last November, German authorities warned that Islamist terrorists were preparing attacks on airports and railroad stations. But German investigators said there was no clear motive in the Frankfurt shootings, although witnesses described Uka shouting “Allahu Akbar!” or “Jihad! Jihad!” before firing on the American military shuttle bus.