VIDEO: Dallas Chief Details Why Gunman was Killed with Bomb, Dismisses Critics

The gunman who killed five Dallas police officers in a shooting spree Thursday night scrawled the letters "RB" on a wall with his blood before he was killed with a remote-controlled robot carrying a bomb, Police Chief David Brown said Sunday.

Brown told  CNN the letters and other markings indicated Micah Xavier Johnson, 25, was wounded in a shootout with police during a protest march Thursday night in downtown Dallas. Brown said investigators were going through Johnson's laptop, journals and cellphones trying to determine the significance of the letters and other scrawlings.

Brown said the Army veteran demanded a black negotiator, which he got. As negotiations dragged on, Brown said he became concerned that Johnson "would charge us and take out many more" officers.

"He just basically lied to us, playing games, laughing at us, singing, asking how many did he get and that he wanted to kill some more," Brown said.

Brown said Johnson was shooting on the move — a tactic he studied at a local self-defense school about two years ago, school founder Justin Everman told the Associated Press.

Johnson was finally cornered behind a brick wall in a parking garage, but Brown determined that any attempt to arrest or shoot him would be too dangerous. Brown said he approved using the bomb, which he described as about one pound of C4 set off with detonating cord, according to USA Today .

He dismissed critics who suggested Johnson could have been subdued with non-lethal weapons such as tear gas. "I don't give much quarter to those who ask these type of questions from comfort and safety away from the incident," Brown said.