DALLAS -- An Army veteran killed by Dallas police after he fatally shot five officers amassed a personal arsenal at his suburban home, including bomb-making materials, bulletproof vests, rifles, ammunition and a journal of combat tactics, authorities said Friday.

The man identified as 25-year-old Micah Johnson told authorities he was upset about the fatal police shootings of two black men earlier this week and wanted to exterminate whites, "especially white officers," officials said.

Law enforcement sources believe Johnson had been planning the attack on police for some time but that it was the police shootings of black men in Baton Rouge, Louisiana, and Minnesota that pushed him over the edge, CBS News homeland security correspondent Jeff Pegues reports.

CBS News has learned that investigators believe Johnson was building his arsenal over the last two years. He was stockpiling guns and gathering the elements to build explosives. Investigators said he had accumulated chemical and electronic precursors to building explosives along with PVC piping.

"This was a mobile shooter that had written manifestos on how to shoot and move, shoot and move, and he did that," Mayor Mike Rawlings told reporters. "He did his damage, but we did damage to him as well."

Get Breaking News Delivered to Your Inbox

Pegues reports those are skills he may have picked up during a six-year military career.

In September 2013, Johnson was activated to serve in the 420th Engineer Brigade in Afghanistan, Pegues reports. He went through basic training, where he would have had to qualify on an M-16 or M-4, which are basic rifles carried by soldiers.

He 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.

In Georgia, Missouri and Tennessee, authorities said gun-wielding civilians also shot officers in individual attacks that came after the black men were killed in Louisiana and Minnesota. Two officers were wounded, one critically.

President Obama and Texas Gov. Greg Abbott asked for the public's prayers. In a letter posted online Friday, Abbott said "every life matters" and urged Texans to come together.

"In the end," he wrote, "evil always fails."

Johnson was a private first class from the Dallas suburb of 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.

A military lawyer says Johnson was accused of sexual harassment by a female soldier when he served in the Army in Afghanistan in May 2014. Lawyer Bradford Glendening, who represented Johnson, said Johnson was sent back to the U.S. with the recommendation he be removed from the Army with an "other than honorable" discharge.

Glendening said Johnson was set to be removed from the Army in September 2014 because of the incident. Instead, Johnson got an honorable discharge the following April - for reasons Gardening doesn't understand.

After the attack, he tried to take refuge in a college campus building and exchanged gunfire with police, the police said.

The suspect described his motive during negotiations and said he acted alone and was not affiliated with any groups, Police Chief David Brown said.

On Saturday, CBS News learned that investigators have found no links between Johnson and violent extremist groups, either foreign or domestic.

Johnson was black. Law enforcement officials didn't disclose the race of the dead officers.

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 were also wounded.

Authorities initially blamed multiple "snipers" for Thursday's attack, and at one point said three suspects were in custody. But by Friday afternoon, all attention focused on Johnson, and state and federal officials said the entire attack appeared to be the work of a single gunman.

With the lone shooter dead, Rawlings declared that the city was safe and "we can move on to healing." He said the gunman wore a protective vest and used an AR-15 rifle, a weapon similar to the one fired last month in the attack on an Orlando, Florida, nightclub that killed 49 people.

When the gunfire began, the mayor said, about 20 people in the crowd were carrying rifles and wearing protective equipment. That raised early concerns that they might have been involved. But after conducting interviews, investigators concluded all the shots came from the same attacker.

In Washington, the nation's top law enforcement official, Attorney General Loretta 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."

The other attacks on police included a Georgia man who authorities said called 911 to report a break-in, then ambushed the officer who came to investigate. That sparked a shootout in which both the officer and suspect were wounded but expected to survive.

In suburban St. Louis, a motorist shot an officer at least once as the officer walked back to his car during a traffic stop, police said. The officer was hospitalized in critical condition.

And in Tennessee, a man accused of shooting indiscriminately at passing cars and police on a highway told investigators he was angry about police violence against African-Americans, authorities said.

Video from the Dallas 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.

Marcus Carter, 33, was in the area when people started running toward him, yelling about gunshots. Carter said the first shot sounded like a firecracker. But then they proceeded in quick succession, with brief pauses between spurts of gunfire.

"It was breaks in the fire," he said. "It was a single shot and then after that single shot it was a brief pause. And then it was boom boom boom boom boom! Pause. Boom boom boom boom boom!"

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

The mayor said one of the wounded officers had a bullet go through his leg as three members of his squad were fatally shot around him.

"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."

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.

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.