(AP) -- The gunman in the deadly attack on Dallas police had plans for a larger assault and possessed enough explosive material to inflict far greater harm, the city's police chief and top elected official said.

Micah Johnson, a black Army veteran, began firing on officers while hundreds of people were gathered in downtown Dallas to protest recent fatal police shootings. Authorities have said the 25-year-old kept a journal of combat tactics and had amassed a personal arsenal at his home that included bomb-making materials.

"We're convinced that this suspect had other plans and thought that what he was doing was righteous and believed that he was going to target law enforcement -- make us pay for what he sees as law enforcement's efforts to punish people of color," Dallas Police Chief David Brown told CNN's "State of the Union" Sunday.

The fact that Johnson had material for explosives and talked of using homemade bombs during a standoff with police before he was killed indicated he could have inflicted more damage with more time, said Dallas County Judge Clay Jenkins.

"If this had not been a crime of opportunity where the protest was quickly organized in response to events in the same week ... he could have caused a lot more harm than he did," Jenkins said.

Five officers were fatally shot in the attacks, and at least nine officers and two civilians were wounded.

Also Sunday, Brown revealed new details about Johnson's negotiations with police, saying that Johnson taunted authorities, laughing at them, singing and at one point asking how many officers he had shot.

Johnson, who served in the Army Reserve for six years starting and did one tour in Afghanistan, insisted on speaking with a black negotiator and wrote in blood on the wall of a parking garage where police cornered and later killed him, Brown said.

The gunman wrote the letters "RB" and other markings, but the meaning was unclear. Investigators are trying to decipher the writing by looking through evidence from Johnson's suburban Dallas home, Brown said.

The writing suggested that Johnson was wounded in a shootout with police. An autopsy will confirm exactly how many times he was hit, Jenkins said.

Authorities do not "have any independent report from an officer saying, 'I think I hit him,'" Jenkins said.

The police chief defended the decision to kill Johnson with a bomb delivered by remote-controlled robot, saying negotiations went nowhere and that officers could not approach him without putting themselves in danger.

Brown said he became increasingly concerned that "at a split second, he would charge us and take out many more before we would kill him."

The shootings just a few blocks from where President John F. Kennedy was slain in 1963 marked the deadliest day for U.S. law enforcement since the Sept. 11, 2001, terrorist attacks.

Federal agents are trying to trace the origin of the weapons used, including a military-style semi-automatic rifle.

About 30 agents are also involved in identifying bullet casings, said William Temple, the Dallas agent in charge for the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The large crime scene includes the parking garage where Johnson was killed and at least two other sites where he is believed to have fired at officers.

Dallas police said Sunday that neither they nor the FBI would confirm that photographs circulating on the internet -- two pictures that show an assault rifle in rubble and the bloodied body of a black man wearing an armored vest amid building debris -- were from the parking garage.

The attack began Thursday evening during protests over the police killings of Philando Castile, who was fatally shot near St. Paul, Minnesota, and Alton Sterling, who was shot in Louisiana after being pinned to the pavement by two white officers.

Video showed protesters marching along a downtown street about half a mile from City Hall when shots erupted and the crowd scattered, seeking cover.

Among those injured were two officers from El Centro College, the school said Sunday night.

El Centro said in a statement posted on its website that Cpl. Bryan Shaw and Officer John Abbott were hurt in Thursday's attack.

Shaw was struck by a bullet as he guarded an entrance to the college, the statement said. The school said Shaw was treated on scene and returned to protect other officers and civilians.

Abbott was also guarding the entrance. The school said he sustained injuries to both legs from flying glass after it was struck by bullets. Abbott tended to his wounds at the scene and then returned to assist others, the statement said.

Both men were resting at home.

Dallas police previously said seven officers and two civilians were hurt in the attack. Its number of wounded did not include any El Centro College officers.