Doug Stanglin, and Trevor Hughes

USA TODAY

DALLAS — Police are uncovering clues, including bomb-making materials, that show the 25-year-old Army veteran, who went on a shooting rampage killing five police officers downtown, carefully plotted his ambush.

Authorities discovered ballistic vests, rifles, ammunition and a journal detailing combat tactics Friday while searching the home of Micah Xavier Johnson in the Dallas suburb of Mesquite, the Dallas Police Department said Friday.

Dallas Mayor Mike Rawlings described the gunman as the lone attacker in the assault late Thursday on police marshaling demonstrators peacefully protesting recent police-involved shootings of African-American men in Louisiana and Minnesota. Seven other officers and two civilians also were wounded.

Police initially reported that at least two snipers were involved in the attack, but Rawlings said investigators later determined Johnson fired from different levels, some elevated above the street. A “written manifesto’’ linked to the gunman offered instruction on shooting techniques and tactical movements, he added.

Rawlings said individuals initially detained after the shooting — who police first characterized as possible suspects — were later determined to be fleeing protesters who were either armed or carrying ammunition gear.

Johnson, who didn't have a criminal record, served six years in the U.S. Army Reserve, including an eight-month tour in Afghanistan, according to the U.S. Army. His Facebook account included the names Fahed Hassen and Richard GRIFFIN or Professor Griff, the Dallas Police Department said in the statement. GRIFFIN, who embraces a "radical form of Afrocentrism," police said, also authored a book titled A Warrior's Tapestry.

As shots rang out late Thursday, and several police officers went down, protesters and bystanders scrambled or fell to the pavement.

The gunman was eventually chased into a building parking lot where negotiators talked to him for almost an hour, Dallas Police Chief David Brown said earlier Friday.

In a press conference Friday evening, Texas Gov. Greg Abbott and Rawlings said the city was recuperating from the trauma of Thursday night.

Dallas and the state of Texas need to "focus on the imperative of unity, that we unite as all citizens of Dallas," Abbott said.

Rawlings told reporters officials were still unraveling the mystery behind what caused the rampage. "This was a man that we gave plenty of options to give himself up peacefully," he said.

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At the earlier news conference, Brown said Johnson was "upset about Black Lives Matter."

"He said he was upset about the recent police shootings. He said he was upset at white people. He said he wanted to kill white people, especially white officers," Brown added.

After talks broke down, the heavily armed gunman was killed by police using a robot-controlled explosive device, Brown said.

Deadliest day for law enforcement since 9/11

The killings marked the deadliest day for U.S. law officers since the Sept. 11, 2001, terrorist attacks, according to The National Law Enforcement Officers Memorial Fund, which tracks on-duty deaths. The fund said the Dallas attack was the 11th ambush-style attack on police officers nationally so far this year, already outpacing a total of eight of these attacks for all of 2015.

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The last major ambush targeting police occurred at a coffee shop in Lakewood, Wash., on Nov. 29, 2009, when a gunman walked in and opened fire on four city police officers working on their laptop computers as they prepared for their work shifts. All four were killed. The gunman later died two days later in a shootout with police.

"Our profession is hurting," Brown said Friday morning. "Dallas officers are hurting. We are heartbroken."

"All I know is this must stop, this divisiveness must stop, this divisiveness between our police and our citizens," he added.

Brown also praised his officers, saying they "put themselves in harm's way to make sure citizens can get to a place of security."

Brown said the ambush was highly organized, indicating the shooter probably knew the march route and "triangulated" the targets from "an elevated position."

'Run for your lives!'

Richard Adams, a bystander, said the protest was "a lovely, peaceful march," until they were walking down Commerce Street near the Bank of America building parking garage when he heard what sounded like "a bunch of firecrackers going off."

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"Everybody just stopped — 'Run, run for your lives!' Women with children and babies and everybody was chaotically running," he told WFAA-TV. "And then, maybe I was a half a block away, calming down a little bit when we heard it again. ... There must have been five times tonight — whenever we thought we were safe, people said 'Run, people were shot!'"

Johnson, wearing heavy body armor and carrying multiple rounds of ammunition, was killed by a police-controlled remote explosive device about 45 minutes after authorities began trying to negotiate with him.

Brown said the suspect, who was holed up in the El Centro community college building, told negotiators, "the end is coming." He also said there were bombs in the area, but officers said none were found.

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Dallas Mayor Mike Rawlings called it a "heartbreaking morning."

"To say that our police officers put their life on the line every day is no hyperbole, ladies and gentlemen, it is a reality," he said. "We as a city, we as a country, must come together, lock arms and feel the wounds that we all feel from time to time."

Brown said his department monitored social media for signs of potential trouble and attended planning sessions ahead of the protest. Before shots rang out, the Dallas Police Department live-tweeted the protest, even posting photos of officers posing with demonstrators. Brown praised the "grit" of his police force as they responded to the mayhem after gunfire broke out.

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Dominique Alexander, founder of the Next Generation Action Network and an organizer of the protest march, condemned the shootings and expressed his condolences to the families of the slain officers.

"We echo the words of President Obama yesterday when he stated, that because black lives matter, it does not mean blue lives do not matter," Alexander s told reporters. "With that being said, we condemn the actions that took lives of five officers and wounded seven. We sincerely thank the Dallas Police Department and the D.A.R.T. police for assisting us with our protest. We understand that although officers volunteered to risk their lives, they certainly do not deserve to die."

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'Vicious, calculated and despicable attack'

President Obama, in Poland for a NATO summit, called the shootings "a vicious, calculated and despicable attack on law enforcement." He told reporters that anyone involved “in these senseless murders will be held fully accountable, justice will be done.”

In emotional remarks at an afternoon prayer service attended by thousands, Rawlings said the community must "attack" racial issues "head on."

"Can we speak against the actions of a relatively few officers who blemish the reputation of their high calling at at the same time, support and defend the 99% of officers who do their job professionally, honestly and bravely?" he asked.

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"I think we can and I think we must," Rawlings said.

Three of the slain officers were identified as Brent Thompson, 43, Michael Krol, 40, and Patrick Zamarripa, 32.

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At Baylor hospital, where several officers staged a vigil for their fallen comrades, dozens of hospital personnel came out and linked arms to shield the grieving officers from the view of bystanders as two bodies were taken out.

Chaotic scene

Live video feeds from news organizations and reports from witnesses painted a picture of a chaotic scene, with police cars converging on a downtown building. Marchers protesting police shootings were moving down Lamar Street near Griffin when shots were fired.

One witness told The Dallas Morning News that he heard "what sounded like six to eight shots."

Theresa Williams told the Associated Press that the injured civilian was her sister, 37-year-old Shetamia Taylor. Williams said her sister was at the protests Thursday night with her four sons, ages 12 to 17.

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When the shooting began, Taylor threw herself over her sons, Williams said. She was undergoing surgery early Friday after being shot in the right calf.

Friday classes were canceled at El Centro College, a community college of the Dallas County Community College District near where the shooting took place.

A call for calm

In Washington, Attorney General Loretta Lynch made brief remarks on what she called a "week of profound grief and heartbreaking loss," referring to both the incidents of police-involved shootings and the killings of five police officers in Dallas.

She said such events can create a sense of "helplessness of uncertainty and of fear"

"These feelings are justified," Lynch said. "But the answer must not be violence."

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The attorney general called for "calm, peaceful, collaborative and determined action" on several fronts to build trust between local communities and law enforcement officers and to create equal justice under the law for all citizens.

She also said the country "must take a hard look at the ease with which wrongdoers can get their hands on deadly weapons and the frequency with which they use them."

Clinton, Trump issue statements

The two presumptive presidential candidates for the two major parties issued statements on the shootings.

Hillary Clinton, who postponed her first campaign appearance with Vice President Joe Biden, said in statement she "mourn(s) for the officers shot while doing their sacred duty to protect peaceful protesters, for their families & all who serve with them."

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Donald Trump issued a statement saying the shootings were "a coordinated, premeditated assault on the men and women who keep us safe." He also referenced this week's police shootings of black men in Louisiana and Minnesota, saying "our nation has become too divided."

"This is a time, perhaps more than ever, for strong leadership, love and compassion. We will pull through these tragedies," he added.

Stanglin reported from McLean, Va. Contributing: Tom Vanden Brook, Kevin Johnson, USA TODAY; WFAA-TV, Dallas-Fort Worth.