Military experts on Twitter have started to dissect footage captured of one of the Dallas shooters as he ambushed police officers during a protest in the city's center on Thursday night.

In the deadliest day for US law enforcement since the September 11, 2001, terrorist attacks, five police officers were shot and killed by snipers during what had been a peaceful demonstration to protest this week's police-shooting deaths in Louisiana and Minnesota.

Seven other officers were injured: Two underwent surgery and three are in critical condition, according to the police.

Malcolm Nance, a combat veteran and retired US Navy Intelligence official, tweeted an analysis of one of the attacker's tactics, caught on video, as he opened fire.

In the graphic video, the attacker — who has unofficially been identified as 25-year-old Micah Johnson — can be seen firing on an unknown individual who had been hiding behind a pillar. It was unclear whether he acted alone.

Nance continued:

"Shooter not using off-hand shots, but good use of cover alternating engag. side. Mainly Double taps ... Shooters intent on surviving. Had escape & evasion plan. Feels militia or Domestic terror grp trained in USA. Initial fusillade was numerous 4-6 shot groups. Mainly 4 shot. Several weapons. Not gang style."

He added: "For anyone who thinks I'm calm. I'm VERY VERY upset. Went to SWAT school with civilian cops. This designed to incite civil war."

Signficantly, a Texas law-enforcement official told CBS News' David Begnaud that Johnson claimed to be a US Army veteran during his standoff with the police. A defense official later confirmed to ABC that Johnson served in the Army Reserve until April 2015.

Sean Parnell, an Army Ranger and combat infantryman who was embedded with the 10th Mountain Division in Afghanistan, said the ambush appeared "sophisticated."

"Heard 3 semi automatic rifles from 3 separate positions," Parnell tweeted. "Coordinated ambush. Fire was synchronized & focused. This was sophisticated."

He added that the attack "definitely takes coordination." "If I'm a betting man," he continued, "they had egress routes pre planned."

Another law-enforcement analyst told CNN: "These were not amateurs."

Dallas police officers near the site of shootings in downtown Dallas early Friday. (AP Photo/LM Otero) Alex Horton, an Iraq veteran turned national reporter for the military news site Stars and Stripes, tweeted a similar analysis.

"Smooth footwork, uses pillar as cover during movement, assaults through, pivots back to previous targets," Horton said. "Takes a cool head. That aggression though, choosing to quickly close the distance to a threat to create a buffer from the video camera's 9 o'clock."

Thomas Gibbons-Neff, a US Marine Corps veteran who now writes for The Washington Post, also weighed in.

"All you can tell is that the shooter appears comfortable doing what he's doing. Controls the weapon, deliberate aiming/firing," Gibbons-Neff said on Twitter.

He added: "I guess what jumps the most is his assault on the officer. In combat, it's called a close ambush. If you're engaged from within 25 yards, you establish fire superiority and assault through the objective. Something — training or no training — this guy clearly did."

Dallas police officers near the scene of the attack on Thursday night. Ron Jenkins/Getty Images Dallas Police Chief David O. Brown said the officers were shot by two snipers in "elevated positions" near the protests and said the department believes the attackers coordinated the ambush.

They "planned to injure and kill as many law-enforcement officers as they could," Brown said. "Some were shot in the back. We believe that these suspects were positioning themselves in a way to triangulate on these officers."

He added that the attackers may have had some prior knowledge of the route the protesters were going to take.

"How would you know to post up there?" he said, referring to the elevated position the snipers were in. "We have yet to determine whether or not there was some complicity with the planning of this, but we will be pursuing that."

The police announced that they had identified four suspects in the attack. Three were in custody, while the fourth was killed by the police after a standoff. Before he was killed by a police-controlled robot bomb, the man told officers he was "upset by Black Lives Matter" and "wanted to kill white people, specifically white police officers," Brown told reporters in a Friday morning press conference.

Brown noted, however, that the suspects in custody were not being very cooperative: "We just are not getting the cooperation we’d like, to know that answer of why, the motivation, who they are."

President Barack Obama addressed the shooting in a press conference from Warsaw, Poland, where he was visiting with NATO leaders, calling it a "vicious, calculated, and despicable attack."

At least 11 officers and one civilian were shot. Four of the officers who were killed were from the Dallas Police Department, while another was from the Dallas Area Rapid Transit agency.