The sniper who fatally shot five police officers in Dallas is believed to have been planning a larger attack, the city's police chief said Sunday, providing new details of how the gunman had been singing, laughing and taunting officers during prolonged negotiations before he was killed by a bomb-equipped robot. Police Chief David Brown said that evidence showed that Micah Johnson, 25, a black Army Reserve veteran who told police that he wanted to kill white officers, had been practicing detonations and that the explosive material had the potential "to have devastating effects throughout our city and our North Texas area."