A former Marine dressed in black and carrying extra ammunition shot and killed three Baton Rouge law enforcement officers Sunday, less than two weeks after a black man was fatally shot by police here in a confrontation that sparked nightly protests that reverberated nationwide. Three other officers were wounded, one critically. Police said the gunman was killed at the scene. The shooting less than a mile from police headquarters added to the tensions across the country between the black community and police. Just days earlier, one of the slain officers had posted an emotional Facebook message about the challenges of...