Dallas shooter Micah Johnson wrote cryptic messages in his own blood

The shooter in the Dallas ambush had been practicing detonating bombs and planned some kind of major attack even before the sniper-style assault in which he killed five police, the city's police chief said Sunday.

Releasing chilling new details of the attack by Micah Johnson, police chief David Brown said Johnson taunted police as he negotiated with them during an hours long standoff -- "playing games, laughing at us, singing" -- asking how many cops he had killed and saying he wanted to take out more.

And at one point, Johnson, apparently wounded, wrote the letters "rb" in his own blood on a wall at the community college where he holed up during the shooting last Thursday. Seven other cops and two civilians were wounded.

Brown said it was not clear what those letters meant.

Johnson, who was black, also insisted on speaking only to a black police officer when he began negotiating with the police, Brown said.

The 25-year-old army vet opened fire with a powerful rifle during a peaceful protest Thursday evening in Dallas against the shooting deaths just days earlier of two black men, in Louisiana and Minnesota.

But a search of Johnson's Dallas-area home after he was ultimately killed by police turned up bomb-making materials and a manual in which he wrote about military tactics.

Police now believe he had been planning something long beforehand, and that the two killings last week were a trigger that prompted him to act, Brown told CNN.

Investigators believe "based on evidence of bomb-making materials and a journal that the suspect had been practicing explosive detonations and that the materials were such that it was large enough to have devastating effects throughout our city and our north Texas area," Brown said.

"We're convinced that this suspect had other plans," he added.

"And we believe that the deaths in Minnesota and the deaths in Louisiana just sparked his delusion to fast-track his plans and saw the protest in Dallas as an opportunity to begin wreaking havoc on our officers," Brown said.

Johnson was a private in the army reserve and had served in Afghanistan.

He knew the route of the Dallas march, and his military training apparently benefited him during the shooting, as he effectively triangulated police and started taking them out with his high caliber rifle, Brown said.