Local media are reporting a suspect involved in a standoff with police in Dallas is dead, after gunmen killed five officers during a protest over the shooting of two black men by police elsewhere in the US.
Eleven officers were shot in what is one of the worst mass police shootings in the country's history.
Police took three people into custody after the shootings on Thursday night (Friday NZT).
The standoff with the other person, in a downtown garage near the El Centro Community College, extended into Friday morning. Gunfire had been exchanged at the garage, police said.
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No motive has been given for the ambush at the downtown protest, one of many held in major cities across the US on Thursday.
Police in New York made more than a dozen arrests on Thursday night, while protesters briefly shut down one of Chicago's main arteries.
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," Mr Brown told a news conference.
Police initially said four officers had been killed but the main union for the Dallas police later reported that one of the seven wounded officers had later died, taking the death toll to five.
"It has been a devastating night. We are sad to report a fifth officer has died," Dallas Police said on Twitter.
The shooting happened as otherwise largely peaceful protests unfolded around the US after the shooting of Philando Castile, 32, by police near St Paul, Minnesota, late on Wednesday. His girlfriend posted live video on the internet of the bloody scene minutes afterward, which was widely viewed.
Mr Castile's death occurred within a day of the shooting of Alton Sterling, 37, in Baton Rouge, Louisiana.
Mr Sterling was killed during an altercation with two white police officers. Graphic video of that incident caused an outcry on social media.
Bomb threats in Dallas
The suspect in the standoff in Dallas told police "the end is coming" and that more police were going to be hurt and killed.
Mr Brown said the suspect also told police "there are bombs all over the place in this garage and downtown".
"This suspect we are negotiating with for the last 45 minutes has been exchanging [gunfire] with us, and has not been very cooperative with the negotiations," Mr Brown said.
A police spokeman later said that no explosives were found in downtown Dallas after primary and secondary sweeps of the area.
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 stand-off was taking place.
"We are being very careful in our tactics so that we do not injure any of our officers or put them in harm's way. We still don't have a complete comfort level that we have all the suspects," Mr Brown said.
"We are leaving every motive on the table on why this happened and how this happened," Mr Brown said.
A large area of downtown Dallas was an active crime scene, Dallas Mayor Mike Rawlings said.
"Our worst nightmare has happened. It is a heartbreaking moment for the city of Dallas," he said.
The Texan city is home to more than seven million people.
Mr Rawlings later 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.
Television footage during and shortly after the shootings showed a heavy police presence, with officers taking cover behind vehicles on the street.
The use of force by police against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York has sparked periodic and sometimes violent protests in the past two years, and has spawned the Black Lives Matter movement.
Anger has intensified when the officers involved in such incidents have been acquitted in trials or not charged at all.