Micah Johnson, the gunman who killed five police officers at a Dallas protest sparked by police shootings of black men was described as a "loner." Here's what you need to know about him. (Victoria Walker,Ashleigh Joplin/The Washington Post)

He looks like any other young man serving his country.

Clear-eyed and serious, Micah Xavier Johnson poses for his U.S. Army portrait in a camouflage cap, the brim pulled level with his thick eyebrows, the image of an American flag flying above his right shoulder.

The photo stands in sharp contrast to the man authorities say opened fire near a peaceful protest in Dallas Thursday night, killing five law enforcement officers.

Johnson, 25, served one tour in Afghanistan between November 2013 and July 2014 and deployed with an engineering unit with which he was listed as a carpentry and masonry specialist; he did not have a combat role.

As police sift through the aftermath of the worst targeted attack against law enforcement officers in modern U.S. history, a fragmented portrait of Johnson is beginning to emerge. So is a long list of questions about his motives, how he obtained his high-powered weapon and how he was able to plan and execute the brazen assault without being detected.

Dallas Police Chief David Brown said Friday that Johnson had told authorities “he was upset about the recent police shootings” and “wanted to kill white people, especially white officers.” Brown said Johnson made the remarks before he was killed by a bomb-carrying robot that had been sent into his location and then detonated by police.

Late Friday, police disclosed more details about their investigations and several more details about Johnson. They said they had searched his home in the suburbs of Dallas, where they discovered bomb-making materials, ballistic vests, rifles, ammunition and a personal journal detailing combat tactics. Police said detectives were poring over the journal entries for more clues to his background and why he carried out the attack.

The investigation has involved numerous federal and local law enforcement agencies. Detectives have interviewed more than 200 people, and police said at least 12 officers returned fire during the chaos not far from Dealey Plaza, where President John F. Kennedy was assassinated by a gunman hiding in the sixth floor of a school book depository in 1963.

Much remains unclear about Johnson’s life, his background and what motivated him to participate in the deadliest attack for U.S. police since Sept. 11, 2001, one that came as protesters were peacefully demonstrating over recent police-involved shootings.

Brown said hostage negotiators did “an excellent job getting the suspect to talk” before determining that Johnson would not surrender.

“The suspect said he was upset about Black Lives Matter,” Brown told reporters Friday. “He said he was upset about the recent police shootings. The suspect said he was upset at white people.”

Patrick Zamarripa, Lorne Ahrens, Michael Smith and Michael Krol of the Dallas Police Department and Brent Thompson of Dallas Area Rapid Transit are the five victims who were killed in Thursday's shooting in Dallas. Seven others were wounded. (Erin Patrick O'Connor/The Washington Post)

Brown continued: “The suspect said he wanted to kill white people, especially white officers. The suspect said we will eventually find the IEDs. The suspect said he was not affiliated with any groups, and the suspect said that he did this alone.”

According to a LinkedIn profile, Facebook and public records, Johnson’s stepmother appears to be white.

Homeland Security Secretary Jeh Johnson and officials in Texas said Friday afternoon that it appeared the attacker did not have accomplices, although Gov. Greg Abbott stressed that authorities still needed to “button down every corner.”

“At this time, there appears to have been one gunman with no known links to or inspiration from any international terrorist organization,” Johnson said.

Police in Dallas also said he had no criminal record.

Micah Johnson left few details about his life on social media. On his Facebook page, its authenticity confirmed by a federal law enforcement official, Johnson is shown in his profile picture raising a fist in the air, a symbol associated with the Black Power movement of the 1960s. He is wearing a dashiki, a colorful shirt widely worn by men in West Africa. Johnson also posted an image of a fist with the text “Black Power.”

He also expressed interest on his Facebook page in the New Black Panther Party, which the Southern Poverty Law Center describes as a “virulently racist and anti-Semitic organization.”

In another Facebook post, Johnson depicts himself in a photo with Richard Griffin, known as Professor Griff of the hip-hop group Public Enemy. As the picture spread on social media Friday, Griffin said on Twitter that he did not know Johnson and he did not “advocate killing cops.”

A woman who identified Johnson as her brother on Facebook wrote in a post on Friday: “I keep saying its not true…my eyes hurt from crying. Y him??? And why was he downtown.”

Later, she added: “The news will say what they think but those that knew him know this wasn’t like him. Only close family can call me. This is the biggest loss we’ve had.”

Efforts by reporters with The Washington Post to reach family members were unsuccessful Friday.

Johnson lived in Mesquite, a Dallas suburb, where neighbors described only fleeting interactions with him.

Jim Otwell said he ran into Johnson in his Mesquite neighborhood last summer. He said Johnson walked up to him and told him that someone had broken into his home and stolen his five assault-style weapons.

Otwell said Johnson invited him into his home, and the two sat at the kitchen table and talked for about 30 minutes while drinking Diet Cokes. He described Johnson as “very nice” and “pleasant” and said Johnson was “very concerned” about the missing weapons.

Otwell, 56, lives across the street from the tan, bricked, two-story home Johnson shared with his mother and younger brother. He said they talked about life in the military. Johnson told Otwell about his days in the Army; Otwell talked about his life in the Navy before he retired.

“I did most of the talking. He was very reserved,” Otwell said.

Otwell said that was the only interaction the two men ever had.

Sandra Johnson, another neighbor, has lived in the area for about 10 years. She said most of the residents there work or are retired. Several Mesquite police officers also live in the neighborhood, one residing around the corner from Johnson’s home, she said.

“This a very nice community,” Johnson said. “Something must have happened to that young man. He was in the military. Maybe he just snapped.”

Micah Johnson graduated in 2009 from John Horn High School in Mesquite, where he was involved in the JROTC program, said Allison Lewallyn, a spokeswoman for the Mesquite Independent School District. Johnson attended John Horn for three years, and his earlier years were spent in the Garland Independent School District, Lewallyn said.

After graduation, Johnson joined the military and was attached to the 420th Engineer Brigade in Seagoville, Tex., serving from March 2009 to April 2015 and attained the rank of private first class.

He is still technically affiliated with the military, records show. He was a member of the Individual Ready Reserves at the time of his death, meaning he did not have to conduct regular training or drills but was available to be called into service.

Keith Alexander in Mesquite, Tex., and Thomas Gibbons-Neff in Washington contributed to this report.