Authorities say 12 Dallas police officers were shot, five fatally, in the nation’s deadliest day for police since the Sept. 11 terror attacks.

A sniper shot from “elevated positions” during a protest against police-involved shootings in the Texas city on Thursday, police said.

Here’s what we know so far about the attack.

The shooting

Shots rang out around 9 p.m. local time Thursday in Dallas near the end of a largely peaceful protest against police-involved shootings, local media outlets reported.

Twelve Dallas officers were shot, five fatally, and two civilians were wounded. Three of the officers shot were women, police said.

Dallas Police Chief David Brown called the incident "a well-planned, well-thought-out, evil tragedy," adding, "We are determined to not let this person steal this democracy from us."

The alleged sniper

The suspected sniper has been identified by a U.S. government source as U.S. Army reservist Micah X. Johnson. The Army confirmed that Johnson served in Afghanistan from November 2013 to July 2014.

Brown said that officers cornered the suspect in a downtown garage and negotiated with him for several hours before “negotiations broke down.” The suspect engaged in a shootout with police before authorities sent a robot carrying a bomb to kill him. “Other options would have exposed our officers to grave danger,” Brown said.

Before his death, the gunman told officers that he was upset about police-involved shootings. “The suspect stated that he wanted to kill white people, especially white officers,” Brown said.

The suspect also told officers that “the end is coming,” Brown said.

Johnson posted a rant against white people last Saturday on the Black Panther Party Mississippi Facebook page. "Why do so many whites (not all) enjoy killing and participating in the death of innocent beings," Johnson wrote in his Facebook post above a graphic video of people participating in a whale-killing, comparing it to the treatment of black people in the United States.

In what appeared to be his own Facebook page, he was portrayed as a black nationalist, with images of Black Power and the red, black and green flag sometimes known as the Black Liberation flag. His profile photo showed him with his clenched fist in the air in the familiar Black Power gesture.

Other suspects?

Dallas Mayor Mike Rawlings said on Friday evening that Johnson had acted alone, though authorities had said Thursday that there were at least two snipers.

"We believe now that the city is safe, and the suspect is dead, and we can move onto healing," Rawlings told reporters at a news conference.

Texas Gov. Greg Abbott said at the news conference however that it remained important to determine whether there were any other co-conspirators to the attack.

The victims

One of the dead officers was identified as Dallas Area Rapid Transit officer Brent Thompson, 43. He’s the first DART officer killed in the line of duty in the department’s history, which dates back to 1989. Thompson had been on the DART force since 2009.

Thompson was married to a fellow DART officer, local television station WFAA reported, citing DART Chief James Spiller. USA Today reported that they married about two weeks ago and that Thompson was also a father and a grandfather from a previous marriage.

Navy veteran Patrick Zamarripa, 32, who served three tours in Iraq, was one of the dead police officers, his family told Reuters. He served in the military reserves as well as worked a Dallas policeman.

His uncle, Hector Zamarripa, said by telephone that Zamarripa was a proud Mexican-American who leaves behind a wife, their toddler-age daughter and a stepson. Although he did not speak much Spanish, he went by the name Patricio among his Spanish-speaking friends and relatives.

"He enjoyed the job, that was his calling," his uncle said .

Also killed was Michael Krol, a 40-year-old officer with the Dallas Police Department, according to a statement from the Wayne County Sheriff's Office in Michigan, where Krol worked as a deputy in the jails from 2003 to 2007.

"We are saddened by the loss of the dedicated officers in Dallas -- one of whom was a former member of this agency -- and also the wounding of the other officers," Wayne County Sheriff Benny Napoleon said in the statement.

Senior Corporal Lorne Ahrens, a 14-year veteran with the Dallas police, was one of those killed, the Dallas Morning News reported. Reuters was not able to reach Ahrens' family for confirmation. He was 48 years old, the newspaper reported.

Michael Smith, 55, also died in the attack, KFDM television in southeastern Texas reported, citing his sister. Smith grew up in Port Arthur, Texas, and served in the military as an Army Ranger before joining the Dallas police in 1989, the television report said.

Reuters was not immediately able to confirm the report.

“We’re hurting,” Brown said at a news conference. “We are heartbroken. There are no words to describe the atrocity that occurred to our city.”

The protest

The Dallas protest was held following the police-involved shootings of two black men – one in Louisiana, the other in Minnesota – just a day apart. 

Alton Sterling, 37, was fatally shot in Baton Rouge, Louisiana, on Tuesday as officers responded to a call that claimed he had threatened someone with a gun. Philando Castile, 32, was shot dead during a traffic stop in St. Paul, Minnesota, on Wednesday.

Quinyetta McMillon, who had a child with Sterling, condemned the attack on police, saying in a statement that "Regardless of how angry or upset people may be, resorting to this kind of sickening violence should never happen and simply cannot be tolerated.”

A Twitter account for Black Lives Matter, the activist group that has long protested the police-involved shootings of black Americans, also spoke out against the attack. "Black Lives Matter advocates dignity, justice and freedom. Not murder,” the account tweeted.

Federal assistance

President Barack Obama, while traveling in Poland, said, "I believe I speak for every single American when I say that we are horrified over these events and we are united with the people and police department in Dallas," he said. He said that the FBI was in contact with local police and that the federal government would provide assistance in the investigation.

U.S. Attorney General Loretta Lynch said Friday that the Justice Department would assist in the investigation into the shooting.

She also said that it has been a “week of profound grief and heartbreaking loss,” citing the Dallas shooting, as well as the shooting deaths of Sterling and Castile.

"Do not let this week precipitate a new normal in this country," she said.