A black US military veteran of the Afghan war who said he wanted to "kill white people" acted alone in a sniper attack that killed five police officers during a Dallas protest decrying police shootings of black men, officials said on Friday.

Seven other police officers and two civilians were wounded in the ambush in downtown Dallas on Thursday night, officials said. Police killed the gunman, identified by authorities as 25-year-old Micah Johnson, with a bomb-carrying robot after cornering him in a parking garage, ending an hours-long standoff.

A search of Johnson's home in the nearby suburb of Mesquite found "bomb-making materials, ballistic vests, rifles, ammunition and a personal journal of combat tactics", Dallas police said in a report on Friday. Police said Johnson had no previous criminal history.

Dallas Mayor Michael Rawlings said Johnson had written "manifestos" on military-style tactics, and social media postings left by Johnson showed he subscribed to a militant black nationalist ideology.

Thursday's attack came at the end of an otherwise peaceful march to protest police killings of two black men this week in Minnesota and Louisiana, the latest police killings of black men over the last two years that have triggered outrage, soul-searching and debates over the role of race.

In Dallas, hundreds of screaming demonstrators ran for safety as police officers patrolling the rally took cover, believing initially that they had come under attack by several shooters.

By late afternoon on Friday, however, investigators had concluded that Johnson, armed with a rifle, was the lone gunman.

"At this time, there appears to have been one gunman, with no known links to or inspiration from any international terrorist organisation," US Homeland Security Secretary Jeh Johnson told reporters in New York.

In Dallas, Rawlings said the shooting "came from one building at different levels from this suspect".

One man was arrested on "unrelated weapons charges" at the scene, and several people were detained for questioning, but police said they were released by day's end on Friday.

Still, Governor Greg Abbott and other officials said they were looking for evidence of any possible co-conspirators.

The ambush marked the highest death toll for U.S. police in the line duty from a single event since the September 11, 2001, attacks.

Gunman angry about police killings

The attack was certain to complicate rising tensions between minority communities and law enforcement following a string of high-profile killings of unarmed black men at the hands of police across the country over the past two years, giving rise to the Black Lives Matter protest movement.

The violence came just over a week before the start of the Republican National Convention in Cleveland, where Donald Trump is expected to become the party's official nominee, and police in Cleveland on Friday tightened their security plan for the convention.

Other police departments across the country, including New York, Chicago and St. Louis, responded to the attack by requiring officers to patrol in pairs rather than alone.

Thursday's attack was especially devastating for the people of Dallas, a city that struggled for decades to heal from the scars left by the 1963 assassination of President John F Kennedy, blocks away in Dealey Plaza.

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

During lengthy negotiations with police, the gunman told police he was angry about the Louisiana and Minnesota killings, Brown told reporters.

"The suspect said he was upset at white people. The suspect stated that he wanted to kill white people, especially white officers," said Brown, who is African-American.

A profile of Johnson's political inclinations also began to emerge. He posted a rant against white people on a black nationalist Facebook group called Black Panther Party Mississippi last Saturday, denouncing the lynching and brutalising of black people.

"Why do so many whites (not all) enjoy killing and participating in the death of innocent beings," Johnson wrote in a post above a graphic video of a whale-killing, comparing it to the treatment of black people in the United States.

In what appeared to be his own Facebook page, Johnson was portrayed with 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 salute.

The US Army said Johnson, 25, had served as a private first class in the Army Reserve and was deployed to Afghanistan from November 2013 to July 2014. It said Johnson served from March 2009 to April 2015 and was a carpentry and masonry specialist with the 420th Engineering Brigade based in Texas.

"Heartache and devastation"

Details on how the shootings unfolded remained unclear. Video of the attack taken by a witness shows a man carrying an assault-style weapon, first crouching then charging at and shooting another person who appeared to be wearing a uniform. That person then collapsed to the ground. Reuters could not immediately confirm the authenticity of the video.

The Rev Jeff Hood, an organiser of Thursday night's protest, said he had been chatting with some of the police officers on the street when gunfire erupted.

"I saw what I believe were two police officers that went down. I didn't know what to do," Hood told reporters on Friday.

President Barack Obama, in Poland for a NATO summit, called the Dallas shootings "a vicious, calculated and despicable attack on law enforcement". Obama, who has been stymied by the Republican-led Congress in his bid for new gun control laws, planned to visit Dallas early next week, at the mayor's invitation, the White House said.

As Dallas reeled from a night of carnage, police came under fire in three other states.

A man in Tennessee opened fire on a highway, killing a woman and grazing a police officer with a bullet on Thursday, because he was troubled by incidents involving black people and law enforcement, authorities said on Friday. Police officers also were wounded in shootings in Missouri and Georgia on Friday, though the motives in both of those incidents was unknown.

Largely peaceful protests unfolded around the United States after the police shooting of Philando Castile, a 32-year-old black man, on Wednesday, during a traffic stop near St Paul, Minnesota. A day earlier, police in Baton Rouge, Louisiana, shot dead Alton Sterling, 37, while responding to a call alleging he had threatened someone with a gun.

Both Hillary Clinton, the presumptive Democratic presidential nominee, and Trump canceled campaign events for Friday following the attack.