Former Blackwater guard Nicholas Slatten leaves federal court in Washington in 2014. (Cliff Owen/AP)

Former Blackwater security guard Nicholas A. Slatten was sentenced to life in prison Wednesday for his role in the 2007 shooting of unarmed Iraqi civilians by U.S. contractor forces in a Baghdad traffic circle, an incident that marked one of the low points of the Iraq War.

U.S. District Judge Royce C. Lamberth in Washington rejected a claim of innocence by Slatten, 35, of Sparta, Tenn., who was convicted in December of first-degree murder for shooting Ahmed Haithem Ahmed Al Rubia’y.

U.S. prosecutors said Slatten fired the first shots in what became an onslaught of unjustified machine gun and grenade fire from a Blackwater convoy into stopped traffic that left 14 civilians dead and 31 wounded in Baghdad’s Nisour Square. The Sept. 16, 2007, massacre triggered a diplomatic crisis and fed deep resentment over the accountability of American security forces.

Slatten, a U.S. Army veteran who served two tours in Iraq, and his defense said he would appeal what they called a wrongful conviction fueled by politics and errors by prosecutors and U.S. justice system.

“I am the victim of these prosecutors who know I am innocent, yet in fear of losing they are willing to kill me [through life imprisonment] for what I did not do,” Slatten said. The sentence came after his third trial in the case. His 2014 conviction was overturned on appeal, and a second trial last summer ended in a hung jury.

Lamberth called statements by Slatten’s family “powerful and important” but rejected his claim of innocence, saying he agreed with jurors that evidence showed that the convoy was not under insurgent fire and that Slatten shot into Al Rubia’y’s vehicle with premeditation, striking the medical student between the eyes.

Lamberth, a U.S. Army captain and lawyer who served in Vietnam, disputed Slatten’s defenders’ claims that his conduct was justified, saying he had seen combat, “but I was in a situation where we depended on each other to carry out orders to ensure that innocent people were not needlessly killed, and we followed those orders.”

U.S. Attorney Jessie K. Liu for the District of Columbia and FBI Washington Field Office chief Timothy Dunham announced the sentence but declined to comment.

Three other Blackwater guards convicted of manslaughter and other charges in the shootings are set to be resentenced by Lamberth on Sept. 5. The 30-year sentences for Paul A. Slough, Evan S. Liberty and Dustin L. Heard, were vacated on appeal last year.

Slatten said his mandatory life sentence for murder was unjust compared with the others, arguing that prosecutors offered him a plea to manslaughter with a sentencing range of five to 10 years in prison, but he said he would not admit to killing someone he did not kill.

Lamberth said “in an ideal world,” a manslaughter conviction would have been a better outcome, but Slatten turned down prosecutors’ offer, “gambled on all-or-nothing” results at trial and would face the consequences.