Nicholas Slatten was convicted in December of first-degree murder by a federal jury in US District Court in Washington, the second time he had been found guilty on the charges.

Slatten was convicted of murdering Ahmed Haithem Ahmed Al Rubia'y, 19, an aspiring doctor who was one of more than a dozen civilians killed by guards of the private security group Blackwater in Baghdad's Nisour Square on September 16, 2007.

While escorting a diplomatic convoy, Blackwater guards opened fire in the bustling square with sniper rifles, machine guns and grenade launchers -- allegedly without provocation -- leaving at least 14 civilians dead and at least 18 wounded. The Iraqi government says the toll was higher.

The shooting deepened Iraqi resentment of Americans in the country four years after US forces toppled dictator Saddam Hussein and raised questions about Washington's expanded use of armed contract guards.

In court for sentencing Wednesday, Slatten remained defiant, calling the decision a "miscarriage of justice that will not stand," according to The New York Times.

It was Slatten's third trial on the charges. His first conviction was thrown out and the jury was unable to reach a unanimous verdict at his second trial in 2018.

Slatten was one of four Blackwater guards who were found guilty in 2014. The 30-year sentences originally given to the others were also vacated for retrial.