A black US army reservist who served in the Afghan war and said he wanted to "kill white people" took part in an attack in which five police officers were shot dead at a protest decrying police shootings of black men, officials said today.
Seven other police officers and two civilians were wounded in the ambush in Dallas last night.
Police killed the gunman, identified as 25-year-old Micah Xavier Johnson, with a bomb-carrying robot after cornering him in a parking lot, ending an hours-long standoff.
In a statement, Dallas police said some people had identified him "as a loner".
Following a search of his home, detectives found bomb-making materials, ballistic vests, rifles, ammunition and what police described as "a personal journal of combat tactics".
The sound of gunfire from a high-powered rifle sent a panicked crowd of hundreds of protesters screaming and running for their lives near the end of a march to protest police killings of black men this week in Minnesota and Louisiana.
The Louisiana and Minnesota shootings, both now the subject of investigations, were the latest in a series of police killings that have triggered protests over police use of force against black suspects and racial disparities in the American criminal justice system.
"This was a well-planned, well-thought-out, evil tragedy by these suspects. And we won't rest until we bring everyone involved to justice," Dallas Police Chief David Brown said.
During lengthy negotiations with police, the gunman said he was angry about the Louisiana and Minnesota killings, Chief Brown said.
"The suspect said he was upset about Black Lives Matter," said Chief Brown.
Black Lives Matter is a movement that protests about the treatment of black people by US law enforcement.
"The suspect said he was upset at white people. The suspect stated that he wanted to kill white people, especially white officers," said Chief Brown.
Authorities said that the gunman fired at least some of the shots in the attack but have not ruled out that other shooters were involved. Details on how the shootings unfolded remained unclear.
It also was not clear how one person could have shot so many officers, though video of the attack taken by a witness shows a gunman carrying an assault-style weapon and carrying large amounts of ammunition.
The video appears to show a man with a rifle crouching at ground level and charging at and then shooting another person who appeared to be wearing a uniform. That person then collapsed to the ground.
Police in Dallas said they were questioning two occupants of a Mercedes they had pulled over after seeing a man throwing a camouflage bag inside the back of the vehicle, which then sped off on a downtown street. A woman was also taken into custody near the garage where the standoff took place.
US President Barack Obama called the shooting "a vicious, calculated and despicable attack on law enforcement".
"As a mark of respect for the victims of the attack on police officers... I hereby order that the flag of the United States shall be flown at half-staff," Mr Obama said in a statement issued as he attended a NATO summit in Warsaw.
Obama: 'There has been a vicious, calculated and despicable attack on law enforcement' https://t.co/Vrf8u3279ohttps://t.co/D0JRA6HLO8
— RTÉ News (@rtenews) July 8, 2016
A string of killings of black men and boys by police in cities including Ferguson, Missouri, New York, Baltimore, Chicago, Cleveland, Tulsa, Oklahoma and North Charleston, South Carolina gave rise to the Black Lives Matter movement against excessive police force.
A Twitter account describing itself as representing the Black Lives Matter movement sent the message: "Black Lives Matter advocates dignity, justice and freedom. Not murder."
The US army said Johnson had served as a private first class in the army reserve, made up of part-time soldiers, 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.
Dallas shooting: Sound of gunfire as police approach the scenehttps://t.co/B02dhUwj1D
— RTÉ News (@rtenews) July 8, 2016
Three of the slain officers were identified today. One was Brent Thompson, 43, who joined the Dallas Area Rapid Transit police department in 2009, according to police.
Another was Patricio Zamarripa, 32, an US navy veteran, according to his family.
Also killed was Michael Krol, 40, according to the Wayne County Sheriff's Office in Michigan, where he used to work.
The Dallas-Fort Worth metropolitan area is one of the United State's most populous cities and is home to more than seven million people.
Both Donald Trump and Hillary Clinton called off campaign rallies in the wake of the attacks, with the Republican presidential contender describing the shootings as "horrific" while noting the "senseless, tragic" recent deaths of Mr Castile and Mr Sterling.
"Our nation has become too divided. Too many Americans feel like they've lost hope," Mr Trump said in a statement.
Mr Trump cancelled a trip to Miami after the tragedy and Mrs Clinton put off a stop in a working-class part of Pennsylvania where she had been due to campaign with Vice President Joe Biden.
I mourn for the officers shot while doing their sacred duty to protect peaceful protesters, for their families & all who serve with them. -H
— Hillary Clinton (@HillaryClinton) July 8, 2016
Before the Dallas attack, Mrs Clinton also voiced concern about the deaths of African-Americans, saying, "too many young black men and women have been taken from us."
