Investigators believe the Dallas gunman who shot 14 people, including five murdered police officers, originally planned a much larger attack.

Dallas' WFAA-TV, citing a law enforcement source with knowledge of the investigation, reported that authorities think Micah Johnson changed his attack plan so the shootings would coincide with Thursday's protest against police brutality.

"We think he was probably planning something bigger based on what we found at [Johnson's] house [in suburban Mesquite]", the source said "He had a bunch of explosive materials. He didn’t have the explosives created."

Among those materials were metal pipes, as well as chemicals that could be used to create pipe bombs. It was not immediately clear whether Johnson had any specific targets in mind for a larger attack.

The source added that Johnson "literally had hundreds and hundreds of rounds in magazines attached to his body [Thursday] so it wasn’t like he was running out of ammunition ... His plan was to kill as many as he could."

Dallas police Chief David Brown said Friday that authorities found bomb-making materials, rifles, ammunition and a "personal journal of combat tactics." Dallas Mayor Mike Rawlings described Johnson, believed to be the only gunman in Thursday's attack, as "a mobile shooter" who had written manifestos on how to "shoot and move — and he did that."

On Saturday, the Associated Press reported that Johnson, an Army veteran who served a truncated tour in Afghanistan, received training from a private self-defense school that teaches firearm tactics, including "shooting on the move."

A person who said he was in charge of the organization told AP that Johnson received instruction at the Academy of Combative Warrior Arts in the Dallas suburb of Richardson about two years ago. The man refused to answer additional questions and would not give his name.

The academy website refers to one of its courses as a "tactical applications program," or TAP.

"Reality is highly dynamic, you will be drawing your firearm, moving, shooting on the move, fixing malfunctions, etc. all under high levels of stress," the website says. "Most people never get to train these skills as they are not typically allowed on the static gun range."

The TAP training includes "shooting from different positions," ''drawing under stress" and "drawing from concealment."

People could be seen training Saturday at the school in a nondescript shopping center, but the doors were locked.

When asked about Johnson, a man who answered the door said, "He trained two years ago. ... I don't know anything about Micah. I'm sorry. He's gone. He's old to us. I have thousands of people."

The suggested timeline of Johnson's training at the academy was corroborated by a police report from May 8, 2015, when someone at a business a short distance away called in a report of several suspicious people in a parked SUV.

The investigating officer closed the case just minutes after arriving at the scene in a parking lot behind a strip mall. While there, the officer spoke to Johnson, who said he "had just gotten out of a class at a nearby self-defense school."

Johnson told the officer he was "waiting for his dad to arrive" and pick up his brother. No one else was apparently questioned.

The Associated Press contributed to this report.