Johnson, who was killed by a police remote-controlled bomb early Friday, had learned about the military in the ROTC program at the high school he attended in Mesquite, a blue-collar suburb east of Dallas. During his military service, he was a private first class with a specialty in carpentry and masonry, according to the military. Officials said he served in the Army Reserve for six years starting in 2009 and did one tour in Afghanistan from November 2013 to July 2014.
When authorities searched Johnson's home Friday they found bomb-making materials, ballistic vests, rifles, ammunition, and a personal journal of combat tactics.
Dallas Police Chief David O. Brown said that Johnson, who was black, had told negotiators before he was killed that he was upset with the fatal shootings earlier in the week of black men by police officers in Louisiana and Minnesota.
"The suspect said he was upset with white people and wanted to kill white people, especially white officers," Brown said.
The African American Defense League posted a message Wednesday encouraging violence against police in response to the shooting in Louisiana. "The Pig has shot and killed Alton Sterling in Baton Rouge, Louisiana! You and I know what we must do and I don't mean marching, making a lot of noise, or attending conventions. We must 'Rally The Troops!' It is time to visit Louisiana and hold a barbeque. The highlight of our occasion will be to sprinkle Pigs Blood! Louisiana Revolutionaries You are being called out! Make ready and we shall come as thieves in the night!" The message was attributed to Dr. Mauricelm-Lei Millere, a leader in the organization.
Leaders of the New Black Panther Party have "long expressed virulently anti-white and anti-Semitic opinions," said the Southern Poverty Law Center. "Its leaders have blamed Jews for the 9/11 terrorist attacks and for the slave trade."
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The Dallas Police Department said that Johnson's Facebook account also included information about Richard Griffin, also known as Professor Griff, who "embraces a radical form of Afrocentrism."
On his Facebook page Friday afternoon, Griffin, a member of Public Enemy - an influential hip hop group known for its politically charged lyrics - said he does "not advocate killing Cops."
There were other hints about Johnson's political leanings, according to his Facebook profile. While photos depict Johnson in his camouflage Army uniform, the cover shot on his Facebook page carries the red, black and green Pan-African flag.
Among Johnson's other social media likes were I Love Black Archaeologist, a web series whose main character uses a time machine to visit famous black people; Black Love Matters; and the Nubian Rootz Cultural Center - groups that focus on the history and accomplishment of African-Americans. The Southern Poverty Law Center said Johnson also "liked" the Nation of Islam and the Black Riders Liberation Party, which it described as "hate groups."
There were certainly other sides to Johnson. He was known by his family and neighbors as an "Army strong" veteran who loved playground basketball.
He was believed to have shared a two-story tan brick home in Mesquite with family members. He graduated from John Horn High School in Mesquite, including membership in ROTC, school district officials said.
Johnson had no criminal history, according to authorities. Local court records show his parents were divorced in 1996.
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A relative had left a positive comment on Johnson's Facebook page celebrating his birthday in 2014, calling him "definitely Army strong" and an "entertaining, loving, understanding, not to mention handsome friend, brother (and) son."
At a news conference Friday, Brown said Johnson told negotiators he had acted alone and was unaffiliated with any group. In the midst of the shootings, authorities had said they believed there was more than one shooter.
After Johnson was killed, a relative posted on her Facebook page, "I keep saying its not true...my eyes hurt from crying. Y him??? And why was he downtown." She did not respond to Facebook messages.
For several hours Friday morning, police blocked access to the home in Mesquite. Investigators wearing Bureau of Alcohol, Tobacco, Firearms and Explosives vests carried out several bags of material.
Just before noon, officers reopened the street. No one answered a knock on the door.
Nearby, friend Israel Cooper said Johnson went by "Xavier,' his middle name. Cooper said Johnson had a "cool vibe, wasn't really political and seemed educated.
Cooper said he and Johnson played basketball at a park near the Johnson home. "He would be out there for eight hours. Like it was his job. Just hoopin'," said Cooper.
Cooper said that when he heard the suspect was Johnson, he "was in disbelief because he's just not like a violent or rough dude."
"So I was, 'Nah, it's probably another Xavier somewhere, you know,' " Cooper said. "But then, with pictures on the internet and stuff, I'm like 'OK.' "
Cooper added: "It's the quiet ones that just do the most devastating stuff. You never see it coming. But then it's more expected, like 'I should have known.'