Four Dallas police officers have been shot dead by sniper fire during protests against the shooting of black men by police, authorities say.
One suspect is in custody after a shootout with police, while a person of interest turned himself in, Dallas police tweeted.
Gunfire broke out as demonstrators marched through the city.
The protests were sparked by the deaths of Philando Castile in Minnesota and Alton Sterling in Louisiana.
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The gunfire broke out at around 20:45 local time on Thursday (01:45 GMT Friday).
Dallas Police Chief David Brown said 11 officers were shot "ambush style" by the sniper fire, killing three. Police later said a fourth officer had died. One of those killed was a transport police officer.
The police said it appeared that two snipers had fired from "elevated positions" during the protest rally.
"We believe that these suspects were positioning themselves in a way to triangulate on these officers from two different perches... and planned to injure and kill as many law enforcement officers as they could," he said.
The mayor of Dallas, Mike Rawlings, said it was a "heartbreaking moment for the city".
Image copyright Dallas PoliceOne of the march's organisers, Rev Jeff Hood, saw people scramble for cover as gunfire rang out.
"I ran away from the shots trying to get people off the streets and I was grabbing myself to see if I was shot," he told the Dallas Morning News.
Police issued a photo of one man with a rifle slung over his shoulder. It is not clear if he was involved in the shootout. Police later tweeted that he had handed himself in.
Dallas police also said officers had intercepted a car after a person threw a camouflaged bag into the back and sped off. Two occupants were being questioned.
Flights over Dallas have been restricted.
Captured on video
The Dallas protests were among several held across the US over the police use of lethal force against African Americans.
Philando Castile was shot dead at a traffic stop in St Paul, Minnesota , on Wednesday, while Alton Sterling was killed by police a day earlier in Baton Rouge, Louisiana.
Both incidents were captured on video, reigniting what has become a national debate.
Image copyright APUS President Barack Obama said "all fair-minded people should be concerned" about the frequent police killings of black Americans.
Pointing to statistics showing African-American citizens are far more likely to be shot by police by whites, Mr Obama called on law enforcement to root out internal bias.
"When incidents like this occur, there's a big chunk of our fellow citizenry that feels as if it's because of the colour of their skin, they are not being treated the same,'' he said. "And that hurts."
But Mr Obama also said there was "extraordinary appreciation and respect for the vast majority of police officers who put their lives on the line to protect us every single day. They've got a dangerous job. It's a tough job."
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