- Authorities aren't sure why the Dallas gunman twice wrote "RB" with his own blood after killing officers
- Report: The gunman's mother says he had wanted to be a cop and became "a hermit" after Afghanistan
(CNN)Shortly after he gunned down five officers in Dallas, Micah Xavier Johnson sang and laughed at police. He threatened more deaths with explosives. And he left a final, cryptic message: the letters "RB," scribbled twice on a wall with his own blood.