DALLAS, July 10, 2016 (Xinhua) -- Two police officers mourn for killed colleagues outside Dallas police headquarters in Dallas, the United States, on July 9, 2016. Five police officers were killed and seven others wounded Thursday by a black gunman who ambushed the police during a black protest in Dallas, Texas to retaliate against white policemen in the wake of the Louisiana killing. This is the deadliest police-killing incident since the Sept. 11, 2001 terror attack. (Xinhua/Zhang Yongxing)
