The moment he heard “officers down,” Dallas Police Maj. Max Geron said, he “knew it was bad.”

The protest against U.S. police violence was supposed to be peaceful, but violence had interrupted the demonstration in downtown Dallas, turning it, he said, into a “hellish situation.”

Gunshots echoed throughout the area, and men and women in uniform fell to the ground as reports came in that a sniper – or maybe snipers – were shooting from up high.

Police, Geron said, were “frantically searching, trying to find out where he, she, they could be.”

“My job then was not to put my car or someone else’s car into the ditch as I worked to get there as quickly and safely as I could,” he wrote. “The adrenaline was coursing.”

Geron penned a chilling, emotional account over the weekend of the horror that hit the city’s police force when a lone gunman shot and killed five officers and wounded several others in the deadliest attack on law enforcement in the U.S. since 9/11.

“In that hellish situation of being fired upon, working tactically to overcome that threat. Being strong and conquering ones fear and staying alive – that is the appropriate place for the warrior in a gun battle,” he wrote in his account, posted on Homeland Security Watch.

“Not a warrior pitted against a segment of the populace – a warrior trying to stay alive against one trying to kill you.”

Earlier in the week, Alton Sterling, a black man, was shot and killed by a white police officer in Baton Rouge. Then, Philando Castile, another black man, was fatally shot during a traffic stop in Falcon Heights, Minn.

Both deaths were broadcast on social media, igniting nationwide outcry and prompting peaceful protests.

People in cities across the country were shouting “hands up, don’t shoot” and waiving signs reading, “Black Lives Matter.”

Then, near the end of the protest Thursday in Dallas, the scene turned bloody and chaotic.

Police swarmed the site of the slayings, searching for a killer.

Geron said he reported to a command post near the convention center, while others hunkered at El Centro Community College, where the shooter, later identified as 25-year-old Micah X. Johnson, had been spotted.

SWAT teams were putting together a plan to try to catch him.

“I posted a quick ‘I’m working and I’m safe,’ message to Facebook because my text and messenger apps were lighting up with inquiries from across the country from friends and relatives watching the horror unfold on television and social media,” Geron wrote in his account. “I began to post information to Twitter to let the community know that we were working hard to protect them and let know what was going on.”

Geron said “the casualty reports started to roll in” – two, then three, then four officers down.

About the same time, he said, some angry protesters started confronting officers at a skirmish line.

“One officer later told me, ‘I tried to tell them that we were there to protect them and the guy said, ‘Protect us hell! You guys are the targets tonight!’ and started laughing,’ ” Geron wrote.

In his most chilling detail from that night, Geron wrote about the moment he saw the cellphone footage showing a fellow officer’s “execution-style killing.”

He wrote:

“The shooter could be seen in front of El Centro college with its distinctive concrete pillars on Lamar St. He was moving and shooting – ducking behind and re-emerging from the pillars shooting at officers. Finally you could see one officer working to engage the shooter move behind one of the pillars. However this time, the shooter was advancing on the officer but the officer didn’t know it. The officer moved to the right and looked down the right side of the pillar just as the shooter rounded the left side of the pillar and from a couple of feet away, shot the officer with an assault rifle. Then he stood over him and executed him by shooting him in the head.

“All of us in the command post visibly recoiled at that sight. It was the stuff of flash picture memories – the kind you have when you can tell someone where you were when men landed on the moon, when the you learned that the Challenger space craft exploded or any other incredibly significant event in your life occurs. In that instant it was indelibly burned into our brains.

“Back to work.”

Soon, the officers received word that Senior Cpl. Lorne Ahrens, a 14-year veteran of the department, had died.

“Upon learning Lorne was among those killed, we all knew we had watched his death shortly before,” Geron wrote. “He was the officer behind the pillar who was ambushed and shot before he could react. He was one trying to protect our freedom to peaceably assemble and was summarily executed for doing so.”

The shooter was ultimately killed by a bomb-equipped police robot.

Geron was relieved by other officers and headed home.

“As I got into my car I glanced again at the messages and immediately the tears hit me but I fought them back,” he wrote. “For a veteran police officer, it can be unnerving to feel like you’re not completely in control of your emotions – I didn’t like it but intellectually I knew at some point I would need to grieve. I just didn’t want to do it sitting in my car at the scene before I had to drive home.”

Geron said he continues to picture his comrade’s murder in his head.

“I keep trying to work out, in my head, how if it had been me behind that pillar, I would have done something different, ANYTHING different and I wouldn’t have been the one to get shot in the back. It’s something that officers do every time we watch a dash cam video. It’s human nature to rationalize away the fear and to help us ‘prepare’ for the next time when it is us. We are guardians absolutely.”

“These men who died,” he added, “were men of guardian hearts protecting citizens’ constitutional rights as warriors in a battle for their lives.”

(c) 2016, The Washington Post · Lindsey Bever