Investigators believe former Army reservist Micah Johnson was planning a much larger attack than the deadly armed assault he carried out in downtown Dallas on Thursday.

They think he moved up his plans for an attack, using the protest rally over controversial police shootings in Minnesota and Louisiana as a target of opportunity.

“We think he was probably planning something bigger based on what we found at the house with the explosive materials,” said a law enforcement source with knowledge of the investigation. “He had a bunch of explosive materials. He didn’t have the explosives created.”

Johnson’s devastating attack Thursday night claimed the lives of four Dallas police officers, one DART officer and injured seven other officers. Two civilians were also shot.

Authorities have seized the car that Johnson drove downtown. The car was found by the white-colored columns where the shooting of one of the officers took place. The car was registered to his mom.

“I think that target of opportunity is what drove him as far as timing,” the source said. “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.”

As families of the fallen officers began to plan funerals, portions of downtown Dallas remained closed Saturday.

Authorities are using surveying equipment to map out the chronology of the deadly shootout. It involves triangulating the location of victims and where shell casings to help them map out the series of events.

Police Chief David Brown said Friday that investigators found bomb-making materials, rifles, ammunition, ballistic vests and a “personal journal of combat tactics” in a search of Johnson’s Mesquite home.

The law enforcement official who spoke to News 8 says that investigators found “numerous metal pipes of various lengths.” They also found various chemicals that could be used to create pipe bombs, he said.

It is not clear where Johnson would have learned how to make explosive devices.

Johnson served in the Army Reserves for six years and was stationed at the Seagoville Reserve Base in North Texas. He also served in Afghanistan. He was a private first class with a specialty in masonry and carpentry.

Investigators are planning to interview Johnson’s former supervisors to learn more about his time in the military.

The protest march was coming down Commerce Street and crossing at Market Street when the gunfire started just before 9 p.m. Thursday. Johnson’s attack unleashed a terrifying scene of chaos as terrified protestors, including women and children, ran for cover and police officers charged toward the gunfire and returned fire.

Video shows Johnson firing from behind white-colored concrete pillars at ground level, then running toward another pillar. A police officer runs up. Johnson surprises him from behind, shooting him from behind.

Police initially believed there had been at least two shooters. That now appears to be because Johnson was employing a tactic of “shoot and scoot.” He’d fire off rounds and then move to another location, making it appear there was more than one shooter.

Investigators believe it’s possible that he was using the vehicle to move from one location to another during the gun battle.

Officers ultimately cornered him in the second floor of a downtown parking garage. He told police during the standoff that he wanted to kill “white people, especially white officers.” He also claimed to have planted other explosive devices in the area. None have been found so far.

Negotiators tried to convince him to give himself up, but he continued to open fire on them.

When it was clear that he would not go without trying to kill or wound more police officers, police used an explosive device on a remote-controlled robot to kill Johnson – the first time a police department in the U.S. has been known to use such a tactic to take out a suspect.

Brown said Friday that investigators did not believe Johnson had any accomplices.

The source told News 8 that investigators had found nothing in his social media to indicate that someone else was involved or knew that he was planning an attack.

Investigators have not yet determined what he may have planned to target in a larger attack. He also had not come on the radar screen of federal agencies prior to Thursday’s deadly attack.