Updated at 7:23 p.m. ET

Officials say a gunman shot and killed five police officers Thursday at a Dallas protest against police shootings of black men, in a bout of violence that didn't end until the suspected gunman was killed by police using explosives delivered by a robot. Seven other officers and two civilians were also injured.

The suspect, who died in a parking garage, was Micah Xavier Johnson, authorities say. Johnson was a U.S. military veteran who had served in Afghanistan, and told negotiators he was upset about police shootings and wanted to kill white police officers.

Law enforcement officers have provided NPR with what they believe is a manifesto from Johnson, which says it should be released after Johnson's death. In the short purported manifesto, the writer points to police shootings of black Americans as his motivation and criticizes the Black Lives Matter movement.

Police, who took three suspects into custody, initially said at least four people were involved in the attack. Officials now describe Johnson as the "lone suspect," though they have not ruled out the possibility of co-conspirators.

Law enforcement officials with knowledge of the investigation tell NPR's Dina Temple-Raston that evidence suggests Johnson acted alone — and that he had been planning the attack for some time.

At a press conference Friday, Dallas Mayor Mike Rawlings said "the city is safe," and healing can begin.

The attack began just before 9 p.m. local time, at the end of a downtown protest march condemning two police killings of black men in other cities earlier this week.

Early in the investigation, police said they believed multiple shooters, whom police described as snipers, took elevated positions in a parking garage so they could "triangulate" their attacks and inflict harm on as many officers as they could. Officials now suspect there was only one shooter, targeting what had been a peaceful event.

"There are no words to describe the atrocity that occurred to our city," Dallas Police Chief David Brown said Friday morning. "All I know is that this must stop, this divisiveness between our police and our citizens."

The attack has been condemned by leaders and supporters of the Black Lives Matter movement, who also call for continued peaceful protests over police shootings, The Associated Press reports.

Here's what we know about the situation. We'll be updating this post as more news comes in:

Suspects

Police took three suspects, including one woman, into custody in the aftermath of the attack. But authorities tell NPR the suspected gunman, who appears to have been working alone, was killed after an hours-long standoff with police at a parking garage at El Centro College in downtown Dallas.

The suspect who was killed is Micah Xavier Johnson, two federal law enforcement officials briefed on the investigation tell NPR.

The Pentagon confirms that Johnson served in the Army Reserve and was deployed to Afghanistan. He had the rank of Private First Class and was a carpentry and masonry specialist. You can read more at our separate page on what's known about Johnson.

Police Chief David Brown said Johnson told negotiators "the end was coming," that he's "going to hurt and kill more" law enforcement, and that there were bombs planted all over the garage and downtown.

Just before dawn Friday, police said their sweeps for explosives had turned up no such devices.

During negotiations with police, the man later identified as Johnson spoke about his motivations, Police Chief David Brown said at a Friday morning news conference.

"He was upset about Black Lives Matter," Brown said, adding that the man cited the recent killings of black people and said that he wanted to kill white people — particularly white police officers.

A search of Johnson's home uncovered "bomb-making materials, ballistic vests, rifles, ammunition and a personal journal of combat tactics," police said Friday.

Three other suspects were initially taken into custody, three men at a traffic stop and a woman by the parking garage.

Another man whose picture had been circulated as a possible "person of interest" in the case turned himself in and was later released; Dallas police had posted a picture of the man in a camouflage T-shirt, carrying a long gun.

Timeline

Hundreds had gathered for the Black Lives Matter rally and march, which was coming to a close just before 9 p.m. CT when gunfire cut through the air.

"They were targeting the police," NPR's Wade Goodwyn reports for Morning Edition. "They let the protesters pass by and started shooting the police."

Part of the gun battle that ensued was captured on a Facebook live stream. We'll warn you that it includes images and language some may find disturbing.

Police believe the shots came from an "elevated position," targeting officers who had been helping secure the rally.

"Dozens of Dallas police cars arrived immediately" after the first shots were fired," reports member station KERA. "Officers were out in force, wearing helmets, shields and holding assault weapons."

In addition to those officers, many of the police who were trapped at the scene were in their patrol and bike uniforms.

For minutes afterward, gunfire echoed through the city's streets. A gunman in a parking garage exchanged fire with police for 45 minutes, according to the Dallas Morning News.

A standoff then ensued, which ended when the suspect was killed just before 3 a.m. local time. Police Chief Brown says the department used a "bomb robot" to place a device "where the suspect was" and detonate it — a move that he said was made to prevent any further risk to police officers.

"Robots have been part of police tactical equipment for years, used to surveil crime scenes with cameras, to defuse bombs or to aid in hostage negotiations by delivering phones or food," NPR technology reporter Alina Selyukh says. "But law enforcement and robotics observers say that this may be the first time a police robot has been used to intentionally kill a suspect."

Victims

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

To read profiles of the victims, see our separate page.

Dallas Area Rapid Transit officer Brent Thompson, 43, was among the slain, according to DART. Thompson had joined the agency in 2009. He is the first officer killed in the line of duty in DART's history.

DART Police Chief James Spiller told CNN that Thompson had gotten married in just the last two weeks.

DART also released the names of three injured officers who are expected to recover. They are Omar Cannon, 44, Misty McBride, 32, and Jesus Retana, 39.

Patrick Zamarripa, 32, was one of the four Dallas Police officers who died in the attack.

He had served in the U.S. Navy, with deployments to Iraq and Bahrain. The Washington Post reports that Zamarripa had a young daughter.

Michael Krol was also killed, NPR's Colin Dwyer reports. Krol, who had worked at a jail system in Michigan, had worked hard to become a police officer, his uncle told Detroit media outlets. He'd moved to Dallas and realized that dream.

Officers Lorne Ahrens, 48, and Michael Smith, 55, were also killed in Thursday's attack.

Most of the officers who were injured last night have now been released from medical care, Brown said, while some still need more treatment.

Praising his city's police department, Mayor Rawlings said Dallas recently had the lowest rate of officer-involved shootings of any large American city.

The five deaths make this the deadliest attack on American law enforcement since Sept. 11, 2001, which the National Law Enforcement Officers Memorial Fund calls "the deadliest day in U.S. law enforcement history."

Witnesses

Protester Wyatt Rosser, who was close to the front of last night's march, told member station KERA about his experience:

"The Black Lives Matter rally ... started out as what was, to me, the biggest and most inspiring rally I've been to in Dallas. The speakers were great and everything was peaceful and beautiful until the end of the march. ...

"There were really beautiful speakers, a lot of great things were said, and it felt really unifying and we were all standing in solidarity, and that happened, and everyone just scattered and we broke apart. That was probably the most intense and disheartening thing for me, how symbolic it was being linked in arms, feeling this really strong moment and then hearing gunshots, and then everything just fell apart."

Rev. Jeff Hood, one of the organizers of the march, said it was always intended to be nonviolent, and that the attack was "not the work of peaceful protesters. This was the work of people who had planned this out very much in advance."

He witnessed the shooting and described the events to NPR:

"We marched and certainly everything was peaceful. The police were commenting on how peaceful and nonviolent it was," he says. Then, close to the end of the march, Hood was talking to a police sergeant when he heard gunshots.

"At first I didn't know what had happened," he said. "Once I realized what had happened I was close enough to where I was rubbing my stomach and rubbing my chest because I thought it was possible that I had been shot. The sergeant ran toward the shooting and I knew that I had over 800 people behind me that I was responsible for, and so I was carrying a black wooden cross and that wooden cross instantaneously became a shepherd's crook. I was screaming, you know, 'Active shooter, active shooter, out of the way ... run, run, run!'

"It's just devastating that these five officers have lost their lives, that they won't be going home to their families this morning. I am devastated, I am angry, I am frustrated," he said.

Rev. Michael Waters was also at the march, and he, too, emphasized the positive relationship between the crowds and the police before the shooting began.

"The police have been a very active and supportive part in supporting activism here in Dallas," he said. "Any time that I personally have been a part of a rally or march or protest, the Dallas Police Department has been there to protect, to ensure that the march or the rally was orderly and to ensure that everyone made it home safely.

On Thursday night, those gathered at the rally had observed a moment of silence and were beginning to return to their cars, Waters says. "People were on their way home," he told NPR. "And it's as we were dispersing that gunshots began to ring out."

Waters says his heart goes out to the families of the officers who were killed. When he preaches on Sunday, he says, he will tell his congregation that "we need each other.

"I'll preach love, I'll preach unity and I'll preach togetherness in our community," he said.

The Aftermath

In addition to the local and FBI investigation efforts, the Bureau of Alcohol, Tobacco, Firearms and Explosives says it has some 30 personnel on the scene.

"Our K9s and explosives enforcement officers are continuing to conduct explosives sweeps for the safety of the public," a spokesman in Dallas tells NPR's Carrie Johnson. "This is the initial stages of a what we expect to be long investigation. We are not disclosing any firearms information at this time."

Police have closed off around 15 blocks of downtown as their investigators work Friday, and they're unsure when the area will be reopened.

A number of downtown office buildings are closed today. They include Dallas County administration buildings, courts and museums.

"Everyone is just stunned, beyond belief," reports NPR's John Burnett, who was born in Dallas.

Dallas is in mourning, reports NPR's Wade Goodwyn, who's based in the city:

"Last night was just devastating for the city of Dallas. You know, it's taken us 50 years to get over the charge that we are the city of hate, after the [President John] Kennedy assassination. And there were some strange similarities. This shooting happened about two blocks away from where the president was killed. Lee Harvey Oswald knew the parade route, and picked an elevated position from which to attack."

Police say that the gunman Thursday night, too, knew the protest route and took an elevated position.

And in both cases, Wade says, the effects were devastating.

Hundreds of people attended an interfaith vigil in Dallas on Friday, mourning the attack.

Responses

President Obama called it "a vicious, calculated and despicable attack on law enforcement," in remarks delivered at the start of his trip to Warsaw, Poland.

Saying that he had spoken with Dallas Mayor Mike Rawlings, Obama said, "I believe that I speak for every single American when I say that we are horrified over these events, and that we stand united with the people and the police department in Dallas."

Obama added that while the "twisted motivations" of the attackers will surely emerge, "there is no possible justification for these kinds of attacks, or any violence against law enforcement."

The president added that the FBI is in contact with Dallas police about the case. Obama will be cutting his trip to Europe short to return to the U.S. a day early, and will be visiting Dallas early next week, the White House announced late Friday.

In a separate news conference, U.S. Attorney General Loretta Lynch said, "Those we have lost this week have come from different neighborhoods and backgrounds – but today, they are mourned by officers and residents, by family and friends – by men and women and children who loved them, who needed them and who will miss them always."

Toward the end of her address, Lynch said:

"And to all Americans: I ask you not to allow the events of this week to precipitate a 'new normal' in our country. I ask you to turn to each other, not against each other as we move forward. And I urge you to remember, today and every day, that we are one nation. We are one people."

At a press conference Friday afternoon, Texas Gov. Greg Abbott said this challenging time calls for unity and resilience from the people of Dallas and the state as a whole.

"We must summon the ability to respond to this challenge in a way that will make Dallas and Texas even stronger," he said.

This is a breaking news story. Some things reported by the media will later turn out to be wrong. We will focus on reports from officials and other authorities, credible news outlets and reporters who are at the scene. We will update as the situation develops.

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