An ex-Marine who caused a terror scare at the Pentagon and Arlington National Cemetery last summer pled guilty Thursday to firing shots at military buildings in the Washington, D.C. area. Yonathan Melaku, then 22, was arrested on June 17, 2011 after he was seen in Arlington National Cemetery at night with a backpack. The backpack allegedly contained a package labeled ammonium nitrate, a common fertilizer than can be used in explosives, spent firearm ammunition and a notebook referencing the Taliban, Osama Bin Laden and al Qaeda.