DALLAS — The suspect in the deadly attack on Dallas police officers scrawled letters in his own blood on the walls of the parking garage where officers cornered and later killed him, the police chief said Sunday. Micah Johnson, a 25-year-old Army veteran, wrote the letters “RB” and other markings, David Brown told CNN’s “State of the Union.” Investigators are looking through evidence from Johnson’s suburban Dallas home to try to figure out what those letters might mean, Brown said. The chief defended the decision to kill Johnson using a robot-delivered bomb, saying negotiations went nowhere and that officers could not approach him without putting themselves in danger. During the roughly two-hour standoff in the garage, Johnson lied to and taunted the police negotiators, Brown said. Johnson had practiced military-style drills in his yard and trained at a private self-defense school that teaches special tactics, including “shooting on the move,” a maneuver in which an attacker fires and changes position before firing again. He received instruction at the Academy of Combative Warrior Arts in the Dallas suburb of Richardson about two years ago, said the school’s founder and chief instructor, Justin J.