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

US District Judge Royce Lamberth of 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.

US 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 Nisur Square. The Sept. 16, 2007, massacre triggered a diplomatic crisis and fed deep resentment over the accountability of American security forces.


Slatten, a decorated US 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 the US 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 US Army captain and lawyer who served in Vietnam, refuted Slatten’s defenders’ claims that his war zone 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.’’