Maybe it was the run-in with police in Richardson a year ago, after someone reported that he looked suspicious sitting in a parked Chevy Tahoe at a strip mall.

Maybe it was his encounter with a famous black-power activist this spring. Or his attendance at a film festival focused on Malcolm X, the charismatic black leader assassinated in 1965.

The truth is that we may never know when or why the slow burn that was Micah X. Johnson's life ignited into a plot to murder white police officers. That plot ended overnight Thursday with the fatal shootings of five Dallas officers, followed by Johnson's death via a police robot bearing a bomb.

Forced out of the Army, Johnson used the skills he'd honed there to unleash bullets on the uniformed police officers with deadly precision.

Friends and acquaintances described the 25-year-old as a nice guy who in recent years cared for a younger brother. But they also say he had an obsession with heavy-duty weaponry and an interest in the military that dates at least to his senior year in high school.

And as he told Dallas officers before negotiations failed, he was enraged by the police shootings of black men elsewhere in the country -- the same deaths that had brought a diverse crowd into the streets of Dallas for what began as a peaceful protest.

Johnson "was always very affected by the police stuff and had very strong feelings about being black," said Caitlyn Lennon, 27, a friend who worked with him at a sandwich shop years ago."I can only imagine how pissed off he would be in the past year, watching all of the shootings."

Just fragments of Johnson's life have become public, and it is too soon to get a clear picture. Family members in Garland and Mesquite declined to talk to the reporters who waited outside their homes on Saturday. In Facebook posts that have since been deleted, Johnson's sister Nicole wrote, "The news will say what they think, but those that knew him know this wasn't like him."

The police investigation remains in its early stages. But officials said Friday that a search of his home in Mesquite had turned up "bomb-making materials, ballistic vests, rifles, ammunition and a personal journal of combat tactics." They said he was described as a loner.

Authorities investigate Micah Johnson's home Friday morning along Helen Lane in Mesquite. A search turned up "bomb-making materials, ballistic vests, rifles, ammunition and a personal journal of combat tactics," police said. (Andy Jacobson/The Dallas Morning News) (The Dallas Morning News)

Johnson told police he had been training for the deadly ambush; one neighboring family said Friday that he had been performing what looked like military training exercises in his yard.

His mother, Delphene, got divorced in 1996, when he was 4. She lives in Mesquite and has worked at a church in DeSoto, according to public records. Her ex-husband lives in Garland.

Johnson graduated in 2009 from John Horn High School in Mesquite, where he participated in JROTC, the high school officer training program, according to the school district. N'Kia Johnson-Williams, who took several classes with him, said he was very smart and not political. At a school with a diverse student body of blacks, whites and Asian-Americans, she said, "I'd never known him to pick particular sides with race," adding that his stepmother was white.

He joined the Army Reserve the same year he completed high school, serving with the 420th engineering brigade.

Johnson also briefly took classes at Richland College, school officials said late Sunday. He attended class in the spring of 2011; he took four courses and did not complete any of them.

His jobs included a stint at Jimmy John's in Richardson. Chris Jennings, who owns the sandwich shop, said in an email that Johnson went to work there in 2011 and "departed of his own accord in 2012."

The following year, he was deployed to Afghanistan for 12 months. Several soldiers who served with him told The Dallas Morning News that they had been ordered not to discuss him.

One family friend said Johnson had changed when he returned from overseas.

"He was withdrawn, didn't want to talk to people anymore, didn't believe in God anymore," said Myrtle Booker, 62, who knew his mother through church.

His interest in guns had grown, she said.

"But all that hating white people -- no, we didn't know any of that."

Police responding to a burglary at his home in August 2014 reported two guns stolen. Jim Otwell, who lives nearby, said Johnson told him about the missing firearms.

"He seemed like a really mellow, laid-back guy," Otwell said. "I swear to God, I don't think I've ever seen him again."

Starting in early 2015, Johnson was an employee at Touch of Kindness, a social service agency that serves children and adults with intellectual disabilities, said Jeppi Carnegie, who owns the business with his mother. Through the job, Johnson cared for his younger brother at home.

"He never talked about anger at all -- he was a normal guy,"Carnegie said. But as a black man, he added, he understands what may have happened.

"When you experience things for so long, sometimes it takes a toll on you, you lash out in ways that may not be the best," he said.

At some point, Johnson appears to have worked at General Dynamics, a defense company with offices in Richardson. A woman who answered the phone there Friday said he'd been an employee. People at the business said employees had been told not to talk to the media. A spokeswoman for the parent company would not confirm or deny his employment.

Pentagon records do not reveal the reason for Johnson's Army discharge in April 2015. But a military lawyer who represented him said Johnson had been accused of sexually harassing a female soldier and had to leave the service. Bradford Glendening, a military attorney who practices near Fort Hood, said that the Army sent Johnson home from Afghanistan, which was unusual. Discipline for sexual harassment is typically counseling, he said.

"He was very much disliked by his command, that was clear," Glendening said.

The woman asked that Johnson receive "mental help" and asked for a protective order for herself and her family, Glendening said, adding that he wasn't sure which type of discharge Johnson ultimately received.

The month after he was discharged, Richardson police received a "suspicious person" report near a strip mall along Greenville Avenue. The caller, whose name was redacted from police records, reported a black 2006 Tahoe with four males inside sitting behind the mall for 20 or 30 minutes before pulling up out front.

Two Richardson police officers arrived and found Johnson inside the Tahoe. He told them he'd just gotten out of a martial-arts class "and was waiting for his dad to arrive" to pick up his brother, the report says.

The officers left after about 10 minutes.

Justin Everman, who runs the martial-arts center, said Saturday that he had not seen Johnson in "at least a couple of years." His Academy of Combative Warrior Arts advertises self-defense classes; an instructor said there was nothing remarkable about the man who became the shooter.

Johnson's passionate interest in the black power movement began to manifest itself on his Facebook page this year. On April 30,he changed his profile picture to a shot of him with Professor Griff, a member of the hip-hop group Public Enemy known for calling out police brutality.

Formed in the early 1980s, the group has always tackled issues of political and cultural significance to the black community.

Griff, whose real name is Richard Griffin, has been a particularly divisive character. At one point, he was expelled from the group after making anti-Semitic and homophobic comments; he later apologized.

On Friday, Griff took to Twitter to say he did not know Johnson.

"I will not sit back and let these people assassinate my character and tie me to the Dallas shootings," he tweeted. He later added, "'No Mr white officer I do not train snipers to kill cops.'"

Johnson subsequently posted emblems of black power and on May 25 changed his profile to a photograph of himself wearing a dashiki with his fist in the air.

By that time he had already attended the May 21 Malcolm X Festival, where he had a brief exchange with Akwete Tyehimba, who owns the Pan-African Connection, an art gallery, bookstore and resource center.

"He seemed like a nice person," she said.

In the wake of new police shootings of black men in Louisiana and Minnesota, she added, "This was not an attack on the police, it was an attack on a system of oppression.

"I cried when I saw what happened in Baton Rouge, the other place, and now here."

Staff writers Holly K. Hacker and Steve Thompson contributed to this story.

Also Andrew Chavez, Tyler Davis, Caleb Downs, Ariana Giorgi, Austin Huguelet, Kevin Krause, Ray Leszcynski, Brittney Martin, Lauren McGaughy, Karina Ramirez and J. David McSwane contributed to this report.