DALLAS — The suspect who killed five police officers in Dallas last week had prepared a larger assault on law enforcement, the city’s police chief said on Sunday, citing evidence gathered from the killer’s home that showed he had practiced with explosives. “We’re convinced that this suspect had other plans,” Police Chief David Brown told CNN, adding that the recent deaths of African-Americans at the hands of police in Minnesota and Louisiana led the Texas shooter to “fast track” his plans. Brown also vigorously defended his decision to use a bomb mounted on a robot to kill the gunman while he was holed up after killing the five officers. He said he approved the plan to detonate a bomb with “about of pound” of C-4 explosives after two hours of fruitless talks with the suspect, who sang, laughed and taunted with police negotiators. The sniper, Micah X.