The gunman who opened fire on police in Dallas said he wanted to kill white police officers and expressed anger at a recent spate of shootings by police before he was killed, reports The Guardian.

The suspect, who has not been named, was cornered for several hours by officers and was killed by an explosive device deployed by a police robot after extensive negotiations failed, said Dallas police chief David Brown.

Brown told reporters at an early morning news conference: “The suspect said he was upset about Black Lives Matter,” during negotiations. “He said he was upset about the recent shootings, he was upset at white people. The suspect said he wanted to kill white people, especially white officers.”

“The suspect said that he was not affiliated with any groups and he stated that he did this alone,” Brown said.

A further three suspects, two males and one female, are in police custody. Authorities declined to provide any further details and it remains unclear if more than one shooter opened fire.

Five police officers were killed and six injured as at least one shooter opened fire on police from an elevated position during a protest in downtown Dallas on Thursday evening.

The shooting marked the deadliest attack on law enforcement officers since 9/11 and threatened to inject yet more tension into the already divisive debate over racial disparities in US policing.

During an emotional statement, Brown paid tribute to the officers injured and killed on Thursday: “We are hurting, our profession is hurting. Dallas officers are hurting. We are heartbroken. There are no words to describe the atrocity that occurred to our city.” 

“All I know is that this must stop, this divisiveness, between our police and our citizens.”

He described officers who responded to the attack as “some of the bravest men and women I know,” adding: “You see video footage after video of them running towards gunfire from an elevated position, with no chance of protecting themselves.”

“Please join me in applauding these brave men and women who do this job under great scrutiny, under great vulnerability, who literally risk their lives to protect our democracy.”

“We do not feel much support most days,” Brown said. “Let’s not make today most days. We need your support to protect you from men like these.”

On Friday morning, about 11 hours after the shooting began, a large swath of downtown Dallas close to Dealey Plaza, where John F Kennedy was assassinated, was closed off with police blocking roads and helicopters circling overhead.

As Dallas begins to come to terms, community leaders are preparing to hold events designed to help the grieving. The organizers of the peaceful march that was attacked are expected to give their account at 11am local time, and an inter-faith service of religious leaders is scheduled for an hour later.

Two of the dead officers have been named as Brent Thompson, 43, a Dallas Area Rapid Transit (Dart) officer, and Patrick Zamarripa. Some of the seven injured officers were in a serious condition. Two civilians were wounded, including Shetamia Taylor, who was shot in the leg while attending the protest with her sons, but her injuries were not thought to be life-threatening.

The anti-violence rally was one of a number being held across the US after the fatal police shootings of two black men, Alton Sterling in Baton Rouge on Tuesday and Philando Castile in Minneapolis on Wednesday.

Barack Obama called the assault a “vicious, calculated and despicable attack on law enforcement” and said the shooters would be held accountable for the “senseless murders”.

Police estimated that about 800 protesters and 100 officers were at the rally. Early in the evening, officers were posing for photographs with demonstrators. But police and witnesses said shots were fired at 8.58pm and the protest descended into chaos. Brown said snipers shot at officers “from elevated positions”.

Video footage caught by local broadcast media and protesters showed the crowd running and screaming once the rapid gunfire broke out. One clip showed an injured officer lying in the street, while another captured one of the shooters opening fire from behind a large pillar.

“Everyone just started running,” Devante Odom, 21, told the Dallas Morning News. “We lost touch with two of our friends just trying to get out of there.”

Charles Wilson, another witness, said police and protestors began gathering at about 5pm and there had also been a small protest on Wednesday.

He said he was close by when the shooting started. “It sounded like firecrackers but it wasn’t firecrackers, it was bullets, and the cops started saying ‘go the other way, go the other way!,” the 55-year-old said.

“It was just sporadic shooting, lots of bullets. I didn’t know who they were shooting at but there was a lot of bullets.”

He walked along the light-rail tracks to evade the scene, as best he could when officers swarmed the area. “It was like everybody was in a box, you couldn’t go nowhere,” he said. “It was basically like something out of a movie.”

Brown told reporters on Thursday night that officers exchanged fire with the trapped gunman into the early hours of Friday, and that the suspect had told officers “the end is coming” and “he’s going to hurt and kill more of us – meaning law enforcement”.

He said that snipers appeared to have positioned themselves on perches at downtown garages during an orchestrated attack and “planned to injure and kill as many law enforcement officers as they could”.

The police chief said the FBI and ATF (Bureau of Alcohol, Tobacco, Firearms and Explosives) had been asked to assist in investigating the bomb threat. The Federal Aviation Authority (FAA) later issued a temporary flight restriction over downtown Dallas.

Brown said that the two male suspects were caught after officers followed their vehicle and recovered two camouflage bags. The female suspect was arrested close to where the standoff had taken place.

The department was investigating whether any of the suspects had prior knowledge of the protest’s planning. “We’re leaving every motive on the table,” Brown said.

The suspect who died was cornered at a parking garage near El Centro College.

Speaking from Warsaw, Poland, Obama said: “I believe I speak for every single American when I say we are horrified over these events and we stand united with the people and the police department in Dallas,” he said. “According to police there are multiple suspects, we will learn more about their twisted motivations but let’s be clear: there is no possible justification for these kind of attacks or any violence against law enforcement.

“Our police have an extraordinarily difficult job and the vast majority of them do their job in outstanding fashion,” he said. “Today is a wrenching reminder of the sacrifices that they make for us. We also know when people are armed with powerful weapons it makes attacks like these more deadly and more tragic.”

Speaking alongside the police chief at the press conference, the Dallas mayor, Mike Rawlings, described the shootings as “our worst nightmare”. He added: “It is a heartbreaking moment for the city of Dallas.”

The Texas governor, Greg Abbott, said in a statement that his “thoughts and prayers” were with the families of those officers who had been shot. “In times like these we must remember and emphasize – the importance of uniting as Americans.”

Seventy-two law enforcement officers died in the 9/11 attacks on the World Trade Center in New York in 2001.

The overnight deaths were the first fatal shootings of US police at demonstrations since unrest spread following the police shooting of the unarmed black 18-year-old Michael Brown in Ferguson, Missouri, in August 2014.

Two officers were wounded by gunfire while guarding the Ferguson police headquarters during a protest in March last year. A 20-year-old man was charged with crimes including first-degree assault and has pleaded not guilty.

In December 2014, two New York police officers were killed in an ambush-style attack by Ismaaiyl Brinsley, who then killed himself. Before the shooting, Brinsley, who had mental health problems, had mentioned on social media high-profile cases of black men being killed by police.