Snipers operating from rooftops in Dallas killed five police officers and wounded six more in a coordinated attack during one of several protests across the country against the killing of two black men by police this week.
Police described Thursday night's ambush as carefully planned and executed and said they had taken three people into custody before a fourth died. Dallas-based media said the suspect died from a self-inflicted gunshot wound after a standoff that extended into Friday morning.
The fourth suspect exchanged gunfire with police during the standoff at a downtown garage and warned of placing bombs throughout the city. Police have not yet confirmed his death.
The attack took place as a protest in Dallas that included a police presence was winding down. It was one of the worst mass shootings of police in U.S. history.
No motive has been given for the shootings at the downtown protest, one of many held in major cities across the United States on Thursday. New York police made more than a dozen arrests on Thursday night, while protesters briefly shut down one of Chicago's main arteries.
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.
Dallas Police Chief David Brown said the shooters, some in elevated positions, used sniper 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.
"It has been a devastating night. We are sad to report a fifth officer has died," Dallas police said on Twitter.
The shooting turned the city's downtown into a sprawling crime scene, unfolding along streets that house major corporations, restaurants and government offices.
President Barack Obama, who was traveling in Poland, spoke to Dallas Mayor Mike Rawlings and expressed his "deepest condolences" on behalf of the American people.
"I believe I speak for every single American when I say that we are horrified over these events and we are united with the people and police department in Dallas," he said.
Obama said the FBI was in contact with Dallas police and that the federal government would provide assistance.
"We still don't know all of the facts. What we do know is that there has been a vicious, calculated and despicable attack on law enforcement," he said.
The shooting, which erupted shortly before 9 p.m. CDT (0100 GMT), occurred near a busy area of downtown Dallas filled with restaurants, hotels and government buildings.
Footage on social media showed terrified protesters running through the streets, gunshots ringing out in the background, and police officers and emergency vehicles swarming the area.
Mayor Rawlings advised people to stay away on Friday morning as police combed the area. Transportation was halted and federal authorities stopped commercial air traffic over the area as police helicopters hovered.
Police said they conducted primary and secondary sweeps for explosives in the downtown area and found none. Large sections of downtown remained closed to the public on Friday morning.
"Our worst nightmare has happened," Rawlings said. "It is a heartbreaking moment for the city of Dallas."
The Dallas-Fort Worth metropolitan area is one of the nation's most populous and is home to more than 7 million people.
The Dallas shooting happened as otherwise largely peaceful protests unfolded around the United States after the shooting of Philando Castile, 32, by police near St. Paul, Minnesota, late Wednesday. His girlfriend posted live video on the internet of the bloody scene minutes afterward, which was widely viewed.
Over the last two years, there have been periodic and sometimes violent protests over the use of police force against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York. Anger has intensified when the officers were acquitted in trials or not charged at all.
'The end is coming"
The suspect in the Dallas standoff had told police "the end is coming" and that more police were going to be hurt and killed. Police chief Brown said the suspect also told police "there are bombs all over the place in this garage and downtown".
Police said they were questioning two occupants of a Mercedes they had pulled over after the vehicle sped off on a downtown street with a man who threw a camouflaged bag inside the back of the car. A woman was also taken into custody near the garage where the standoff was taking place.
"We are leaving every motive on the table on why this happened and how this happened," Brown said.
Mayor Rawlings visited the wounded at Parkland hospital, the same hospital where President John F. Kennedy was taken after he was shot in Dallas in November 1963.
“Dallas had a tragedy when President Kennedy was shot here in the 60s and this is as close to that feeling I think as the city has had in decades," Texas Attorney General Ken Paxton told CNN. “This is going to have a profound effect on Dallas and the people of north Texas for a long time to come.”
Outside, officers stood in formation and saluted as bodies of the officers were about to be transported.