Dallas police said Friday they still have many leads to process as they try to assess what prompted a person or persons to shoot 12 police officers the night before as a march protesting police violence against African Americans came to an end.

Although police were still not prepared to lay out the attackers' motives, the fact that one of the shooters told police he wanted to kill white policemen amid a rally decrying violence by white police officers against black men raised questions.

Dallas residents woke up to discover that five of those officers had died. Additionally, two civilians were wounded in the attack, which began with officers being mowed down by sniper fire and then turned into street-level fighting between officers with handguns and shooters with automatic weapons.

Dallas police chief David Brown referred to an "ambush-style" attack in one of the initial press conferences after the attack.

"At 8:58 [pm (0158 GMT Friday) our worst nightmare happened," said Dallas Mayor Mike Rawlings.

The siege's final hours focused on one suspect who had holed up in a parking garage near the city's El Centro College, engaging officers in an hours-long gunfight.

Police eventually used a robot outfitted with a bomb to kill the suspect, Brown said.

"We saw no other option than to use our bomb robot and to place a device on its extension for it to detonate where the suspect was," he said. "Other options would have exposed our officers to grave danger."

The suspect told a negotiator that he acted alone and that his goal was to kill white people, specifically white police officers, Brown added.

"Our hostage negotiator did an exceptional job getting this suspect to talk before he died," said Brown. "The suspect said he was upset about Black Lives Matter, police shootings, white people. The suspect said he wanted to kill white people, especially white officers."

Black Lives Matter is a US campaign that alleges that US police officers are prone to using excessive violence against African Americans. The group staged several rallies across the US Thursday - including the one in Dallas - after incidents this week in which blacks were killed by white police officers in circumstances deemed suspicious.

During the stand-off, the suspect claimed to have planted bombs and said they would be discovered after his death.

Police also reportedly had three other suspects in custody. No information was released about two of them. Rawlings said the third was a light-skinned African American woman.

Earlier in the night, Brown had said that at least two snipers had coordinated the attacks.

"We believe that these suspects were positioning themselves in the way so as to triangulate on the officers from two different garages and then to injure and kill as many officers as they could."

Witnesses speaking to local media reported panic after the initial shots, interspersed with individuals encountering gunfights between officers and heavily armed shooters.

Some reported seeing the shooters unfazed by police gunfire thanks to their body armour. Others reported seeing the attackers shooting officers in the back or shooting officers who had already been felled.

Brown said he would not release any information about the suspects because he wanted to keep any possible accomplices guessing about what authorities know.

"We're not satisfied that we've exhausted every lead," he said. "So, if there's someone out there who was associated with this, we will find you and we will prosecute you."

City officials warned that large parts of the city would remain roped off Friday as much of the city centre remains an "active crime scene."

The attacks were "vicious, calculated and despicable," said US President Barack Obama, speaking from Warsaw, where he was due to attend a NATO summit.

"We will learn undoubtedly about their twisted motivations," he said, referring to the attackers. "But let me be clear there is no justification for these senseless attacks. ... Justice will be done."

Obama also called for prayers for the wounded before noting that the attacks once again raised the need to debate gun control in the United States.

"We also know that when people are armed with powerful weapons, unfortunately, it makes attacks like these more deadly and more tragic. In the days ahead, we are going to have to confront those realities as well," he said.

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