At least two snipers opened fire on officers from “elevated positions,” according to an initial statement by the city’s police department. Police searched the downturn area for bombs but found none, according to Max Geron of the Dallas Police Department.

Primary and secondary sweeps for explosives are complete and no explosives found. #dallasshooting

— Chief Max Geron (@ChiefGeron) July 8, 2016

The situation is still developing, and details continue to emerge. Here’s what we know so far:

What happened

The first shots rang out around 8:45 p.m. on Thursday near a street in downtown Dallas where the protesters were marching less than a mile from City Hall and police were engaged in crowd-and-traffic control. Several of the officers were injured in the ensuing gunfight. Dallas Police chief David O. Brown was quoted by the Associated Press (AP) as saying that three officers are in critical condition.

Brown said the officers were shot “ambush style,” some of them in the back, according to the Dallas Morning News. Carlos Harris, a local resident who witnessed the incident, told the newspaper that the shooters “were strategic. It was tap tap pause. Tap tap pause.”

The shooting now ranks as the deadliest targeted attack on U.S. police officers in nearly 100 years and the deadliest day for law enforcement since the 9/11 terror attacks, according to the National Law Enforcement Officers Memorial Fund.

None of the protesters, who scattered in search of cover immediately after the shots rang out, have been reported dead. However, the city’s mayor, Mike Rawlings, said one member of the public was wounded, the AP reports, with a relative claiming it was a protester shielding her children from the gunfire.

Theresa Williams said her sister, Shetamia Taylor, was at the protest with her four sons, aged 12 to 17. Williams said Taylor threw herself over her children when the shooting began, and was undergoing surgery after suffering a gunshot wound to the calf.

Dallas authorities announced Friday morning that large parts of the downtown area have been shut down to bring the situation under control, and released a map detailing road closures.

In a speech from Poland’s capital Warsaw, where he is currently attending the NATO Summit, U.S. President Barack Obama called the mass shooting a “vicious, calculated and despicable attack on law enforcement.”

“I believe that I speak for every single American when I say that we are horrified over these events,” Obama added.

The suspects

In a press conference shortly after midnight, Brown said police were currently engaged in a gun battle with a suspect at a parking garage downtown. He added that the suspect told negotiators that “the end is coming” and that he plans to “hurt and kill” more members of law enforcement. He also reportedly claimed that bombs had been planted across downtown Dallas.

Brown also added that a “female who was in the same area” has been taken into custody. “We still don’t have a complete comfort level that we have all the suspects,” the police chief said.

Another person was reportedly spotted carrying a camouflaged bag to a waiting vehicle, which then sped off. The vehicle, a black Mercedes, was subsequently stopped and both its occupants are being questioned, police said.

An individual whose photograph was circulated on Twitter by the Dallas Police Department (and who appeared in unverified footage purportedly taken during the shooting), turned himself in earlier.

The suspect was later named as Mark Hughes and released by police.Hughes told CBS that he contacted police when he was told he was a suspect. He was interrogated for 30 minutes, and released. Hughes said that after his photo was posted by police, he had no idea that they viewed him as “a person of interest”. He told CBS: “The crazy thing is that I could not get to my vehicle because of the roadblocks. In hindsight, I could have been shot.”

The identities and motives of the alleged attackers have not yet been revealed.

The victims

Dallas Area Rapid Transit (DART) identified one of its police officers, 43-year-old Brent Thompson, as being among the victims. Thompson had reportedly joined the DART Police Department in 2009, and the agency said he is the first officer killed in the line of duty since the police department was established in 1989.

“Our hearts are broken,” the statement by DART said. The agency added that three of its other officers wounded in the attack are expected to recover.

Dylan Martinez tweeted that his stepbrother Patrick Zamarripa was one of the victims.

https://twitter.com/KDylanMartinez/status/751314300245905408

The other three deceased are all believed to be Dallas police officers, but their identities have not yet been released to the public.

The bigger picture

Thursday’s shooting could further polarize a heated ongoing national debate about race and law enforcement following the deaths of Philando Castile in St. Paul, Minn., and Alton Sterling in Baton Rouge, La. — both black men shot by police officers within a day of each other.

The protest in Dallas was one of multiple demonstrations that took place across the country against the two police shootings, with hundreds taking to the streets in New York City, Chicago, Philadelphia and other cities.

Aside from the families of the two victims, activists, celebrities and politicians have all spoken out against the deaths of Sterling and Castile.

“We have seen tragedies like this too many times,” President Obama said in an earlier speech from Warsaw following the twin shootings. “When incidents like this occur, there’s a big chunk of our fellow citizenry that feels as if because of the color of their skin they are not being treated the same.”

This article is published in partnership with Time.com. The original version can be found here.