"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."
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.
He 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.
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.
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 left the Army Reserve in April 2015. Military sources said he was transferred to the Individual Ready Reserve, meaning he was no longer connected to a unit or required to continue drilling.
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."
The aunt declined to go into detail about Johnson's 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."