Emergency responders administer CPR to a patient at the Baylor University Medical Center on July 8. Photo: msn.com

It was the deadliest day for US law enforcement since 9/11.

There was a lengthy standoff between police officers and one man suspected to be one of the shooters, before, according to numerous local reports, he was shot dead.

It was unclear whether the suspect was shot by police or killed himself. The Guardian was unable to confirm the reports. 

Three other suspects were in custody.

The Dallas police chief, David Brown, said he did not have a “complete comfort level” that all suspects had been apprehended.

The suspect involved in the standoff had warned that bombs had been planted across the city and a “suspicious package” was also found near his location. The FBI is among authorities investigating the bomb threat.

Officers exchanged gunfire with the trapped shooter into the early hours of Friday, Brown said, telling reporters the suspected gunman had told officers “the end is coming” and that “he’s going to hurt and kill more of us – meaning law enforcement”.

One of the officers killed was named as Brent Thompson, a Dallas Area Rapid Transit (Dart) officer, aged 43. Dart said he was the first of their officers to have been killed in the line of duty.

The anti-violence rally descended into chaos after what Brown described as “two snipers” shot at officers “from elevated positions”. 

Four officers were initially reported shot dead and several of the wounded were said to be in surgery or a critical condition. One later died, according to the Dallas police officers’ association.

Brown 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 at a press conference that the 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 two suspects were apprehended after officers followed their vehicle and recovered two camouflage bags. 

Another suspect, a female, was arrested close to the garage where the standoff was taking 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. 

Dart announced on Twitter that four of its police officers had been struck, one fatally. 

The injuries to the other three officers were described as “not life-threatening”.

Dan Patrick, the lieutenant governor of Texas, said on Twitter: “Our thoughts and prayers are with the police officers who were shot in Dallas tonight.”

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.” 

Protests had taken place around the US earlier on Thursday evening after the fatal police shootings of two African American men, Alton Sterling in Baton Rouge on Tuesday and Philando Castile on Wednesday in Minneapolis.

As large protests grew in cities including New York and Minneapolis, a similar demonstration in Dallas appeared to be proceeding peacefully and in good spirits.

Police estimated that about 800 protesters and 100 officers were present. 

At 7.12pm local time, officers were posing for photographs with demonstrators. However, police and witnesses said that at 8:58pm shots rang out.

Video footage captured by local television cameras and witnesses on social media showed at least one officer on the ground. Protesters were seen fleeing through the streets of downtown Dallas screaming as multiple shots rang out in quick succession.

Graphic video appeared to show one gunman exchanging fire with a man who appeared to be a police officer. The gunman appears to hide behind a pillar outside a building and jump out behind the officer, gunning him down. 

“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.”

Dart suspended all downtown bus and rail service. Streets appeared to have been cleared by officers while they searched for suspects.

The deaths of the officers were the first fatal shootings of police at demonstrations since unrest spread across the US after police shot dead of an unarmed black 18-year-old 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.