A gunman riding on the back of a motorcycle shot to death an American teacher in the city of Taiz, Yemen, on Sunday, a high-ranking government official there told NBC News. The official said the teacher, an adviser at a Swedish-affiliated institute, died immediately after he was struck by gunfire Sunday morning. Taiz is about 50 miles south of the Yemeni capital of Sanaa. Officials from the institute identified the victim as Joel Shrun and said he was born in 1983. However, a U.S. State Department official later told NBC News it had not yet been able to confirm the...