Dallas shooter Micah Johnson wrote cryptic message in his own blood on wall before he died

Investigators found explosive-making materials in his home

AP

Police are trying to decipher the meaning of a cryptic message left by the suspect in the Dallas shooting amid reports that he was planing an additional attack on a much larger scale.

Micah Johnson, a black army veteran, shot at police officers from rooftops killing five and injuring seven during the Thursday night protest. Two civilians were also injured.

During two hours of negotiations with officers in, he taunted authorities, laughing at them and at one point asking how many officers he had shot, according to the city's police chief.

Johnson 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, David Brown told CNN's State Of The Union.

Upon searching Johnson’s home, police found bomb-making materials, and deduced that he had moved up the attack to coincide with the protests – which were in response to the high profile police killings of Alton Sterling in Louisiana and Philando Castile in Minnesota.

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

Mr 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".

“We think he was probably planning something bigger based on what we found at the house with the explosive materials,” a source close to the investigation told ABC affiliate WFAA. “He had a bunch of explosive materials. He didn’t have the explosives created.”

The source continued: “I think that target of opportunity is what drove him as far as timing. … He literally had hundreds and hundreds of rounds in magazines attached to his body so it wasn’t like he was running out of ammunition. He definitely was ready to go and intending to inflict more damage.

“His plan was to kill as many as he could.”

Police killed Johnson using explosives attached to a bomb squad robot after negotiations had failed.

Law enforcement originally suspected two shooters were present at the attack, but Chief David Brown said that Johnson, who was an Army reservist, had studied “shoot and scoot” tactics, where he would change locations quickly after firing off a series of rounds.

On Friday, Dallas Mayor Mike Rawlings described Johnson as "a mobile shooter" who had written manifestos on how to "shoot and move".

Authorities have said the gunman kept a journal of combat tactics and had amassed a personal arsenal at his home, including bomb-making materials, rifles and ammunition.

Johnson donned a protective vest and used a military-style semi-automatic rifle for the shootings.

In all, 12 officers were shot, five fatally, just a few blocks from where President John F Kennedy was assassinated in 1963.