First of Murdered Dallas Cops Identified as Marine Veteran

He was a newly married grandfather, a former U.S. Marine and a veteran cop who had gone to Iraq and Afghanistan to help train local police. But Brent Thompson died protecting the rights of fellow Americans to criticize his brethren in blue.

The 43-year-old Thompson, who joined the Dallas Area Rapid Transit police department in 2009, was the first identified of the five city police killed in a coordinated sniper attack Thursday night in a protest against police brutality. The other names, as well as several who were wounded, are expected to be released Friday morning, following the nation's deadliest day for law enforcement since 9/11.

Related: Obama: America Is Horrified at Worst Attack on Police Since 9/11

"Our hearts are broken," DART spokesperson Morgan Lyons said. "This is something that touches every part of our organization."

Officials said Thompson had married a fellow police officer just two weeks ago.

A dozen police officers were shot and five killed in the attack, in which at least two snipers are believed to have fired at police as they worked one of several protests around the nation sparked by racially charged police shootings in Louisiana and Minnesota.

As details emerged about Thompson's life and service to his community and country, it was clear that his loss would leave a major void. After serving in the Marines, Thompson later worked for a private Pentagon contractor that hired and trained Iraqi and Afghan police in the concepts of "democratic policing." The instruction included training in how to avoid an ambush, according to his LinkedIn page.

"I was responsible for the day to day operations conducted by our American police officers who trained and mentored the Iraqi Police," he wrote.

He also served as an instructor at the Dallas police academy.

"While teaching at the Police Academy, I instructed courses to new recruits on different subject matter, as well as instruction to veteran police officers on state mandated courses," Thompson wrote.

In a 2006 story by The New York Times, Thompson discussed leading an eight-man DynCorp "saturation" training team to Lashkar Gah, in Afghanistan.

Thompson was the first DART cop to be killed in the line of duty since the department was formed in 1989. The other four police officers killed in the attack were from the Dallas Police Department, which had not yet released their names.

Three other DART officers were wounded, but they are expected to recover, Lyons said. They were identified as officers Omar Cannon, 44; Misty McBride, 32; and Jesus Retana, 39.