In her Mesquite, Texas home, about 20 kilometres east of Dallas, the next-door neighbour of Micah Johnson spoke quietly about the man she remembered as a sweet kid, but who she said became a different person after his time in Afghanistan.

The neighbour, who says she has known Johnson since he was a teenager, did not want to be identified, but agreed to speak briefly about the 25-year-old, now responsible for the deadliest attack on law enforcement in the city's history, and the deadliest day for law enforcement in the U.S. since the Sept. 11, 2001 attacks.

"He was a good kid. He was not the same when he came back from Afghanistan," she said.

Johnson changed, she said. He was quieter, more reclusive and withdrawn, and would talk about the horrors of war. Before his tour, he was sweet and quiet, and would help other kids in the community, a neighbourhood she described as mixed, where people of all races and religions live.

"And that's why everybody is so shocked."

Johnson lived in this Mesquite, Texas home with other members of his family. (Mark Gollom/CBC)

For six years starting in 2009, Johnson served in the Army Reserve as a private first class with a specialty in carpentry and masonry, the military said.

According to The Associated Press, on May 2014, six months into his Afghanistan tour, he was accused of sexual harassment by a female soldier and sent home.

Police say Johnson, who was killed by a robot armed with a bomb, had declared he wanted "to kill white people, especially white police officers." His Facebook page revealed that he "liked" groups that included the African American Defence League and the New Black Panther Party, which was founded in Dallas.

Johnson also "liked" the Nation of Islam and the Black Riders Liberation Party, which the Southern Poverty Law Center described as "hate groups."

In a statement, police said Johnson had no criminal history and that "others had identified him as a loner."

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Since the shootings, his home had been been the scene of an intensive investigation launched by police, who found bomb-making materials, ballistic vests, rifles , ammunition and a personal journal of combat tactics.

A bouquet of flowers with a note offering prayers for the family was left at the doorstep of Johnson's residence.

But on Saturday, his residence and the neighbourhood was relatively quiet, save for a scattering of media.

A bouquet of flowers was left on the doorstep of Johnson's home with a note that read: "Praying for your family. Remember this too shall pass. Praying for your comfort and peace."

'I even cried for him'

"I cried as much for the police officers as I did for his family," said his neighbour. "I even cried for him because what was that kid going through that he would do such a horrific thing? A loving kid, a loving family, what was he going through that triggered him?"

More details emerged Saturday about Johnson's background with weapons' training. A neighbour reported to investigators that Johnson had been seen practicing some sort of military drill in his backyard, said Clay Jenkins, the Dallas County judge, the county's most senior elected official.

Johnson also received instruction at the Academy of Combative Warrior Arts in the Dallas suburb of Richardson about two years ago, the school's founder and chief instructor, Justin J. Everman told the Associated Press.

The private self-defence school teaches special tactics, including "shooting on the move," in which an attacker fires and changes position before firing again.

Mike Rawlings, the mayor of Dallas, on Friday said officials believed Johnson was the lone shooter, and that he'd been on the move between locations.