A former guard for Blackwater, the controversial US security contractor, was found guilty of murder on Wednesday for his role in a notorious massacre of unarmed civilians in central Baghdad in 2007.

Nicholas Slatten, 35, was convicted of first-degree murder by a federal jury in US District Court in Washington after five days of deliberations.

Slatten was convicted of killing Ahmed Haithem Ahmed Al Rubia’y, 19, an aspiring doctor who was one of more than a dozen civilians murdered by Blackwater guards in the Iraqi capital’s Nisour Square on September 16, 2007.

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

The massacre fueled growing resentment of Americans in Iraq four years after US forces toppled dictator Saddam Hussein and raised serious questions about the controversial use of armed contract guards by the US government.

The US Attorney’s Office presented testimony from 34 witnesses during the trial, including four who came to the US from Iraq to testify.

According to the government’s evidence, Slatten, of Sparta, Tennessee, was the first guard to open fire.

No date was set for his sentencing. The US Attorney’s Office said the murder charge calls for a mandatory sentence of life in prison.

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.

Slatten was one of four Blackwater guards who were found guilty in 2014. He was originally sentenced to life in prison while the three others were given 30-year prison sentences

An appeals court has ordered that the three other Blackwater guards be resentenced. They are currently in custody pending resentencing.

– with reporting by Agence France-Presse