DALLAS – A gunman who served in the U.S. Army Reserve and said he was “upset at white people” was part of an attack in which five Dallas police officers were killed and another seven were wounded, authorities said on Friday.

The attack ended hours later when police used a robot carrying a bomb to kill the shooter, the Dallas police chief said.

The Thursday night killings, at the end of a protest over this week’s pair of fatal shootings of black men by police in Louisiana and Minnesota, fueled a national debate over excessive police force. The overnight attack raised fears that others would seek to retaliate against police.

Authorities said that the gunman fired at least some of the shots in the attack but have not ruled out that other shooters were involved.

The shooter,  identified as Micah X. Johnson, a member of the U.S. Army Reserve.

A string of killings of black men by police in cities including Ferguson, Missouri, New York, Baltimore and Chicago have given rise to the Black Lives Matter movement against excessive police force. The killings have spurred almost two years of largely peaceful street protests.

Thursday’s shooting sent protesters running in panic while swarms of police found themselves under attack by what they believed to be multiple gunmen using high-powered rifles at ground level and on rooftops.

During lengthy negotiations with police, the gunman said “the end is coming,” according to Dallas Police Chief David Brown.

“He said he was upset about the recent police shootings. said Brown, who is black. “The suspect said he was upset at white people. The suspect stated that he wanted to kill white people, especially white officers.”

Brown declined to say how many people took part in the attack. “We’re going to keep these suspects guessing,” he told reporters at City Hall.

Police said they were questioning two occupants of a Mercedes they had pulled over after seeing a man throwing a camouflage bag inside the back of the vehicle, which then sped off on a downtown street. A woman was also taken into custody near the garage where the standoff took place.

Reverend Jeff Hood, an organizer of Thursday night’s protest in Dallas, said he had been chatting with some of the police officers on the street when gunfire erupted.

“Immediately, when I heard the shots, I looked up and I saw what I believe were two police officers that went down. I didn’t know what to do,” Hood told reporters on Friday. “If we continue to turn to violence, we are going to continue to see heartache and devastation.”

A Twitter account describing itself as representing the Black Lives Matter movement sent the message: “Black Lives Matter advocates dignity, justice and freedom. Not murder.”

With Thursday’s attack, 26 police officers have been shot and killed in the United States so far this year, according to the National Law Enforcement Officers Memorial Fund. That is up 44 percent from the same period in 2015.