A police officer aims his gun at demonstrators and journalists during a protest against police killing of 18-year-old Michael Brown and another 23-year-old black man in Ferguson, Missouri, the United States, on Aug. 19, 2014. A 23-year-old man was shot and killed after he brandished a knife at police officers in northern St. Louis on Tuesday, local authorities said. (Xinhua/Ting Shen)