US President Barack Obama delivers remarks on the police shootings in Dallas after meeting with EU leaders at the NATO Summit in Warsaw, Poland July 8, 2016.[Photo/Agencies]

DALLAS - At least one sniper in Dallas killed five police officers and wounded seven more in a coordinated attack that ended when police used a bomb to kill a shooter who told them he wanted to kill white officers, authorities said Friday.

The attack came during one of several protests across the United States against the killing of two black men by police this week, the latest in a long string of killings that gave rise to the Black Lives Matter movement.

Police described Thursday night's ambush as carefully planned and executed and said they had taken three people into custody before killing the fourth after a long standoff in a downtown garage.

"We had an exchange of gunfire with the suspect. We saw no other option but to use our bomb robot," Dallas Police Chief David Brown told reporters at City Hall.

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

The attack came in a week that two black men were fatally shot by police officers in Baton Rouge, Louisiana, and outside Minneapolis. The killings, both now the subject of official investigations, inflamed tensions about race and justice in the United States.

The shots rang out as a protest in Dallas was winding down, sending marchers screaming and running in panic through the city's streets.

It was the deadliest day for police in the United States since the Sept. 11, 2001, attacks on New York and Washington.

A total of 12 police officers and two civilians were shot during the attack, Dallas Mayor Mike Rawlings said. Three of the officers who were shot were women, he said.

Rawlings told CBS News the people in custody, including one woman, were "not being cooperative" with police investigators. He said the assailant who was dead was being fingerprinted and his identity checked with federal authorities.

Police were still not certain they knew all of the individuals involved in the attack, Rawlings said.

There was no sign of international links to the attacks, U.S. officials said on Friday.

One of the dead officers was identified as Brent Thompson, 43. He was the first officer killed in the line of duty since Dallas Area Rapid Transit formed a police department in 1989, DART said on its website. Thompson joined DART in 2009.

Earlier, Brown said the shooters, some in elevated positions, used rifles to fire at the officers in what appeared to be a coordinated attack.

"(They were) working together with rifles, triangulating at elevated positions in different points in the downtown area where the march ended up going," Brown told a news conference, adding a civilian was also wounded.

A video taken by a witness shows a man with a rifle crouching at ground level and shooting a person who appeared to be wearing a uniform at close range. That person then collapsed to the ground.

Reuters could not immediately confirm the authenticity of the video.