SEATTLE — A U.S. citizen detained in North Korea for 15 months has been returned to a labor camp, prompting worries about his health, his sister said Friday. Kenneth Bae, who led tour groups in North Korea, was arrested in late 2012 and sentenced to 15 years hard labor for unspecified hostile acts. Calls for his release on humanitarian grounds have gone unanswered. After he lost 50 pounds, he was moved last summer to a hospital from a prison work camp where he had been farming vegetables.