DALLAS -- An Army veteran killed by Dallas police after the sniper slayings of five officers during a protest march told authorities that he was upset about the police shootings of two black men earlier this week and wanted to exterminate whites, "especially white officers," officials said Friday.

CBS News confirmed that the suspect has been identified as Micah Xavier Johnson, 25. Johnson was killed by a robot-delivered bomb after the shootings, which marked the deadliest day for U.S. law enforcement since the Sept. 11, 2001, terrorist attacks. In all, 12 officers were shot.

Two law enforcement sources told CBS News that the operating theory at this point was that Johnson was the only gunman, CBS News senior investigative producer Pat Milton reports.

The sources said that the ricochet of bullets made it appear like there was more than one sniper, Milton reports. The investigation was being treated as an active shooter case.

Graphic shows Dallas shooters movements during ambush CBS News

A law enforcement source told CBS News that Johnson had a SKS semi-automatic assault rifle and a handgun, Milton reports. The suspect also wore body armor, the source said.

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On Friday, police officers went to Johnson's home in the Dallas suburb of Mesquite, Texas and removed items. They later said that they found bomb-making materials and a journal of combat tactics in the home.

Johnson was a private first class from Mesquite with a specialty in carpentry and masonry. He served in the Army Reserve for six years starting in 2009 and did one tour in Afghanistan from November 2013 to July 2014, the military said.

After the attack, he tried to take refuge in an El Centro College campus building and exchanged gunfire with officers, police said.

The suspect described his motive during negotiations, Police Chief David Brown said.

"The suspect said he was upset about Black Lives Matter," Brown told reporters. "He said he was upset about the recent police shootings. The suspect said he was upset at white people. The suspect stated he wanted to kill white people, especially white officers."

Brown said he acted alone and was not affiliated with any groups. Investigators believe Johnson had sympathies with black separatist movements, but they have found no links at this point to international or domestic terrorism groups, Milton reports.

Brown blamed "snipers" for Thursday's attack, and authorities initially said three suspects were in custody and the fourth dead. Hours later, officials were vague and would not discuss details.

Sources said it appeared that three people taken into custody in the aftermath of the attack were not connected to the shooting, Milton reports.

Johnson was black. Law enforcement officials did not immediately disclose the race of the dead officers.

Brown said his department and their families were grieving and that the divisiveness between police and the public must stop.

"We don't feel much support most days," Brown told reporters. "Let's not make today most days. Please, we need your support to be able to protect you from men like these who carried out this tragic, tragic event."

A law enforcement source told CBS News that monitoring of social media overnight indicated an increase of threats to police online with one post saying "kill cops," Milton reports.

In Washington, Attorney General Loretta Lynch, the nation's top law enforcement official, told reporters that the Justice Department, including the FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives and the U.S. Marshal's Service, was working with state and local authorities.

"We intend to provide any assistance that we can to investigate this attack and also to help heal a community that has been severely shaken and deeply scarred by an unfathomable tragedy," Lynch said.

The bloodshed unfolded just a few blocks from where President John F. Kennedy was slain in 1963.

The shooting began Thursday evening while hundreds of people were gathered to protest the killings in Baton Rouge, Louisiana, and suburban St. Paul, Minnesota. Brown told reporters that snipers fired "ambush-style" on the officers. Two civilians also were wounded.

Authorities said they were not sure they had located all possible suspects, but attention on Friday quickly turned to the man killed in the parking garage.

Around midday, investigators were seen walking in and out of a home believed to be Johnson's in Mesquite.

None of the other suspects was identified, and the police chief said he would not disclose any details about them until authorities were sure everyone involved was in custody.

Lynch called for calm, saying the recent violence can't be allowed to "precipitate a new normal."

Lynch said protesters concerned about killings by police should not be discouraged "by those who use your lawful actions as a cover for their heinous violence."

It appeared the shooters "planned to injure and kill as many officers as they could," Brown said.

Video from the scene showed protesters marching along a downtown street about half a mile from City Hall when shots erupted and the crowd scattered, seeking cover. Officers crouched beside vehicles, armored SWAT team vehicles arrived and a helicopter hovered overhead.

Demonstrations were held in several other U.S. cities Thursday night to protest the police killings of two more black men: A Minnesota officer on Wednesday fatally shot Philando Castile while he was in a car with a woman and a child, and the shooting's aftermath was livestreamed in a widely shared Facebook video. A day earlier, Alton Sterling was shot in Louisiana after being pinned to the pavement by two white officers. That, too, was captured on a cellphone video.

The Dallas shootings occurred in an area of hotels, restaurants, businesses and some residential apartments only a few blocks from Dealey Plaza, the landmark made famous by the Kennedy assassination.

The scene was chaotic, with officers with automatic rifles on the street corners.

"Everyone just started running," Devante Odom, 21, told The Dallas Morning News. "We lost touch with two of our friends just trying to get out of there."

Carlos Harris, who lives downtown, told the newspaper that the shooters "were strategic. It was tap, tap, pause. Tap, tap, pause," he said.

Video posted on social media appeared to show a gunman at ground level exchanging fire with a police officer who was then felled.

Mayor Mike Rawlings said one of wounded officers had a bullet go through his leg as three members of his squad were fatally shot around him.

The five Dallas officers slain Thursday, July 7, 2016 CBS News

"He felt that people don't understand the danger of dealing with a protest," said Rawlings, who spoke to the surviving officer. "And that's what I learned from this. We care so much about people protesting, and I think it's their rights. But how we handle it can do a lot of things. One of the things it can do is put our police officers in harm's way, and we have to be very careful about doing that."

Few details about the slain officers were immediately available.

Four of the dead were with the Dallas Police Department, a spokesman said. One was a Dallas Area Rapid Transit officer. The agency said in a statement that 43-year-old officer Brent Thompson, a newlywed whose bride also works for the police force, was the first officer killed in the line of duty since the agency formed a police department in 1989.

"Our hearts are broken," the statement said.

Patrick Zamarripa served as a Dallas police officer after first serving in the Navy, where his family said he did three tours in Iraq. Zamarripa, who would have turned 33 next month, was married with a toddler and school-age stepchild.

Michael Krol, 40, was a caring person and had always wanted to help others, his mother said Friday.

Michael Krol is pictured in a handout photo. Wayne County Sheriff's Office/Handout via REUTERS

"He knew the danger of the job but he never shied away from his duty as a police officer. He was a great caring person and wanted to help people. A wonderful son, brother, uncle, nephew and friend," said Susan Ehlke, from Redford, Michigan, in a prepared statement.

Krol's family said in a statement that he moved to Dallas to become a police officer in 2007 because Detroit wasn't hiring. He was a deputy at the Wayne County jail before the move.

Meanwhile, family members told the Detroit Free Press that Krol was single with no children, but had a girlfriend in Dallas. He graduated from the Dallas Police Academy in 2008.

"He was a guy that was serving others," said Brian Schoenbaechler, Krol's brother-in-law. "And he gave his life in service of others."

The other two officers were identified by the Dallas Morning News as Lorne Ahrens and Michael Smith.

Theresa Williams said one of the wounded civilians was her sister, 37-year-old Shetamia Taylor, who was shot in the right calf. She threw herself over her four sons, ages 12 to 17, when the shooting began.

Other protests across the U.S. on Thursday were peaceful, including in New York, Atlanta, Chicago and Philadelphia. In Minnesota, where Castile was shot, hundreds of protesters marched in the rain from a vigil to the governor's official residence.

President Obama said America is "horrified" by the shootings, which have no possible justification. He called them "vicious, calculated and despicable."

Speaking from Warsaw, Poland, where he was meeting with leaders of the European Union and attending a NATO summit, the president asked all Americans to pray for the fallen officers and their families.