DALLAS — Police Chief David Brown says a suspect in the overnight attack that killed five police officers, wounded seven others and wounded two civilians said he was upset over the recent police shootings of black men and wanted to kill white people.

Brown said at a news conference Friday that the suspect made the comments before he was killed by an explosive used by police.

He says his department and their families are grieving and that the divisiveness between police and the public must stop.

Authorities say snipers opened fire on police officers during a peaceful protest in downtown Dallas Thursday night over the recent fatal shootings of black men in Louisiana and Minnesota.

Authorities say three other suspects were arrested.CLICK HERE for photos

The White House said President Obama, who is at a NATO summit in Warsaw, had been briefed on the events in Dallas.

Investigators were sweeping downtown Dallas for possible explosives and sweeping the shooting scenes for evidence, including spent shell casings, CBS Dallas reported. Authorities said no explosives were found. Much of downtown Dallas was still considered a crime scene, Mayor Mike Rawlings told reporters. Officials said many buildings there would be closed Friday and urged people to stay away from that part of the city.

CBS Dallas points out it was the deadliest attack on police officers in the U.S. since 9/11.

CBS Dallas posted an image of officers saluting their fallen comrades as their bodies were being transported from the hospital:

DART issued a statement saying, “As you can imagine, our hearts are broken. This is something that touches every part of our organization. We have received countless expressions of support and sympathy from around the world through the evening. We are grateful for every message. Thank you.”

DART identified its officers wounded in the shooting as Omar Cannon, 44, who joined DART PD in July 2009, Misty McBride, 32, who joined DART PD in March 2010, and Jesus Retana, 39, who joined DART PD April 2006.

The snipers shot down at the officers from an “elevated position,” Dallas police chief David Brown said in a statement. He raised the question of how the snipers knew the route of the protest in order to position themselves.

“They planned to injure and kill as many law enforcement officers as they could,” Chief Brown said.

Dallas police conducted a massive mobilization in the downtown after the shootings, creating a massive crime scene. Buses and public transportation were shut down, stranding hundreds of people in downtown Dallas.

Early Friday, Chief Brown said three subjects were in custody and that police were still in negotiations with a man with whom they exchanged gunfire in a downtown parking garage. That man said “the end is coming” and that he wanted to hurt police. The man also said there were bombs scattered in the downtown. The police bomb squad has been called to handle at least one suspicious package.

Police were negotiating with a gunman in this downtown parking garage early Friday, June 8, 2016 KTVT DALLAS

Earlier, police released on Twitter a man who they said was a person of interest. That man turned himself in and was later released. Police also arrested two men in camouflage gear who were seen in the area in a Mercedes.

Chief Brown said they were concerned that there may be other suspects still at large.

Officers huddle behind squad cars after a shooting at police protest in downtown Dallas, Thursday, July 7, 2016. KTVT DALLAS

The shoots were fired at at approximately 8:45 p.m. CST on Thursday night, as hundreds of protesters marched in downtown Dallas to protest recent police shootings in Minnesota and Louisiana. The Dallas protests were one of various protests held across the nation Thursday night in New York, Washington, D.C., Miami, Baton Rouge and other cities.

According to KTVT, the officers were shot around the intersection of Market and Commerce in downtown Dallas.

“At first I thought it was a firecracker,” one witness, Clarissa Myles, told CBS Affiliate KTVT. “I saw at least 30 shots go off.”

“I knew it was a high caliber rifle, just from the pause and sound itself,” one witness, Jamal Johnson, told KTVT. He described terrified protesters rushing to use a bus station for cover as shots rang out.

Another witness said it appeared the shots came from the roof of a parking garage.

Earlier tonight, in cities across the country, protesters pounded the pavement to express their heartbreak, fury and frustration over the shooting deaths of two black men, Alton Sterling and Philando Castile, this week. Video footage of both murders, shared widely on the internet, has helped narrow the emotional distance the American public usually feels in police shootings of black Americans.

In St. Paul, Minnesota — where Philando Castile was killed during a routine traffic stop with his girlfriend watching — hundreds marched to the Governor’s Mansion alongside the victim’s mother, Valerie Castile, who is just one day into mourning her son’s death, CBS Minneapolis affiliate WCCO reported.

Minnesota Governor Mark Dayton has said that, in his opinion, police officers acted “way in excess” in shooting and killing Castile and would not have done so if he was white. The moment after Castile’s shooting were captured by Castile’s girlfriend, Lavisha Reynolds, who turned on Facebook’s live streaming feature seconds after her boyfriend was shot and narrated the entire encounter to her personal network.