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The sniper who shot a dozen Dallas police officers, killing five, is an Army veteran and a "loner" who had no criminal record but a pile of weapons and bomb-making materials in his house, officials said.

Micah Xavier Johnson, 25, left the Army Reserve in 2015, nine months after returning from a deployment to Afghanistan. He was working as an aide for mentally challenged children and adults before Thursday's mass shooting, according to an employment application obtained by NBC News.

An aunt told NBC News that she believes Johnson — who told a hostage negotiator he wanted to "kill white people," according to authorities — was driven to attack by recent incidents in which black men were killed by cops.

"I think a person can only take so much," she said, adding, "It should not have happened. Nobody wants to see that kind of tragedy."

Dallas Police said Johnson did not have a criminal history. "Others have identified him as a loner," police said in a press released.

Johnson's Facebook page had few public posts but did contain a photo of him raising his fist in what could be a black-power salute and a graphic image of the fist logo. Law enforcement sources said they had found no ties between Johnson and extremist groups, though his Facebook page showed he had liked the pages of black nationalist organizations.

Police noted there was also a photo of Professor Griff, a member of the rap group Public Enemy, who said on Twitter that he did not know Johnson and was angry his name was being associated with the tragedy.

Micah Xavier Johnson, suspect who was killed after a standoff in the ambush of 12 Dallas police officers on Friday. via Facebook

Johnson grew up in Mesquite, Texas, a suburb of Dallas, and graduated from John Horn High School in 2009. He joined the Army Reserve out of school and trained to be a carpentry and masonry specialist after basic training.

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In a January employment application, he said he was looking for construction work.

“I can assist or lead the building of homes or building for the safety of individuals, families or corporations,” he wrote. “I was specifically trained in laying the foundation, framing, interior finishing, exterior finishing, electrical wiring, and plumbing to complete desired building.”

While serving the reserves, Johnson worked for Fly Guys Distribution Company as a "foreman,” managing a team that distributed advertising flyers in the McKinney area, his application says.

In November 2010, he started as shift manager at Jimmy John's Gourmet Sandwich Shop in North Dallas. Two years later, he took a job as a "quality assurance specialist" at the International Truck Plant in Garland, Texas, according to the application. At the plant, he said, he also worked on Army vehicles, including bomb-resistant trucks known as MRAPs.

None of the companies where Johnson said he worked responded to requests for comment.

There is no evidence that Johnson had a security clearance. He was on active duty from September 2013 to April 2015. The military said he deployed to Afghanistan from November 2013 to July 2014.

Micah Xavier Johnson, suspect who was killed after a standoff in the ambush of 12 Dallas police officers on Friday. via Facebook

During his deployment, he did construction work on military bases, he told prospective employers. He also served general guard duty, but there is no evidence that he participated in any combat.

Johnson received an honorable discharge from the Army Reserve in April 2015, military sources told NBC News. Military sources said he was transferred to the Individual Ready Reserve (IRR), meaning he was no longer connected to a unit or required to continue with drill.

At the time of his death, Johnson remained in the IRR. Despite allegations — including reports that he sexually harassed a female soldier in Afghanistan — military sources told NBC News he was not convicted of any criminal offense, and if he had anything less than an honorable discharge, he would not have been permitted to join or serve in the Reserves.

NBC News has not confirmed the sexual harassment allegations.

Since returning from Afghanistan, Johnson had been working for a Mesquite company, Touch of Kindness. He said his job was to “assist [sic] mentally challenged children and adults with transportation to and from various appointments and retail stores.”

A woman who answered the phone at the company told NBC News that Johnson had not come into work on Friday because it was his normal day off. She did not know he was connected to the Dallas shootings and said she found it impossible to believe.

"He's not that kind of person," he said. "He's not violent. That can't be him."

When investigators searched Johnson's home in Mesquite, they found "bomb making materials, ballistic vests, rifles, ammunition, and a personal journal of combat tactics," Dallas Police said. "Detectives are in the processing of analyzing the information contained in the journal."

Johnson's aunt declined to go into detail about his background, including whether he had a history of violence or emotional problems.

"Of course, someone did go out of their mind briefly," she said.

"Everybody is shocked, everybody is so shocked. But everybody sees what this is about. Did Xavier go too far? Yeah."