DALLAS - The gunman who killed five police officers during an anti-violence protest in Dallas at a protest march had practiced military-style drills in his yard and trained at a private self-defense school that teaches special tactics, including "shooting on the move," a maneuver in which an attacker fires and changes position before firing again.
Micah Johnson, an Army veteran, received instruction at the Academy of Combative Warrior Arts in the Dallas suburb of Richardson about two years ago, said the school's founder and chief instructor, Justin J. Everman.
Everman's statement 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 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.
CBS News has learned that investigators believe Johnson began building his arsenal over the last two years, and therefore started right around the time he attended the combat school. He was stockpiling guns and gathering the elements to build explosives. Investigators said he had accumulated chemical and electronic precursors to building explosives along with PVC piping.
Law enforcement sources believe Johnson had been planning the attack on police for some time but that it was the deadly shootings last week of black men by police in Louisiana and Minnesota, that pushed him over the edge, CBS News homeland security correspondent Jeff Pegues reports.
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 25-year-old gunman kept a journal of combat tactics and had amassed a personal arsenal at his home, including bomb-making materials, rifles and ammunition.
Dallas Police Chief David Brown said on CNN on Sunday he believes Johnson was plotting larger attacks against police than what he carried out. Brown told CNN's "State of the Union" on Sunday that Johnson wrote lettering in blood before heading upstairs and writing more in his own blood. He says the 25-year-old Army veteran wrote the letters "RB," and that investigators are looking through things found in his suburban Dallas home to try to figure out what he may have meant by that.
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." Everman declined to specify which classes Johnson took.
"I don't know anything about Micah. I'm sorry. He's gone. He's old to us. I have thousands of people," Everman told The Associated Press on Saturday.
The two men, however, were friendly and talked in Facebook conversations in August 2014. Everman knew Johnson had been out of the country. Army officials said he had been deployed in Afghanistan around that time.
Everman suggested that Johnson "let me know when you make it down this way."
"Will be great to get you back in the academy," Everman said, according to a comment thread saved by the AP before Johnson's Facebook profile was taken down.
"I concur!" Johnson replied.
More recently, a neighbor reported to investigators that Johnson had been seen practicing some sort of military drill in his backyard in the Dallas suburb of Mesquite, said Clay Jenkins, the Dallas County judge, the county's most senior elected official.
Tensions were still high Saturday in Dallas, where 20 square blocks of downtown remained cordoned off as a crime scene. The police department tightened security Saturday evening after receiving an anonymous threat.
Earlier in the day, President Barack Obama called Johnson a "demented individual" who does not represent black Americans any more than a white man accused of killing blacks at a church in Charleston, South Carolina, represents whites.
"So we cannot let the actions of a few define all of us," Obama said from Warsaw, Poland, where he attended a NATO summit.
The president planned to visit Dallas in a few days and to convene a White House meeting next week with police officers and community and civil rights activists.
It was the third time in as many days that Obama has spoken about the fatal police shootings of black men in Louisiana and Minnesota that were immediately followed by the sniper attack in Dallas.
Johnson, who donned a protective vest and used a military-style semi-automatic rifle, was killed by a robot-delivered bomb Thursday after the shootings, which marked the deadliest day for U.S. law enforcement since the Sept. 11, 2001, terrorist attacks.
In all, 12 officers were shot just a few blocks from where President John F. Kennedy was slain in 1963.
Johnson was a private first class with a specialty in carpentry and masonry. He served in the Army Reserve for six years starting in 2009 and did one tour in Afghanistan from November 2013 to July 2014, the military said.
The attack began Thursday evening while hundreds of people were gathered to protest 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.
Marcus Carter, 33, was in the area when people started running toward him, yelling about gunshots. Carter said the first shot sounded like a firecracker. But then they proceeded in quick succession, with brief pauses between spurts of gunfire.
"It was breaks in the fire," he said. "It was a single shot and then after that single shot, it was a brief pause," followed by many shots in quick succession.
After shooting at the Dallas officers, Johnson tried to take refuge in a parking garage and exchanged gunfire with police, authorities said.
During negotiations, he said he wanted to exterminate whites, "especially white officers," the police chief said.