A suicide car bomber killed at least 25 people and wounded 15 on Sunday outside a United States base in Afghanistan s southeastern province of Khost, the site of a suicide attack in 2009 that left seven C.I.A. operatives dead. Provincial security officials in Khost said the car bomber had intended to penetrate the base, Camp Chapman, a major facility at the height of the American military presence, also known as Forward Operating Base Chapman.