Washington federal judge Royce Lamberth declared a mistrial in the case of Nicholas Slatten, 34, after a jury reported last week they were at an impasse and could not reach a unanimous verdict.

Slatten was one of four armed guards for the former US security contractor Blackwater Worldwide who were found guilty in 2014 of murder for the notorious killings in Baghdad's Nisour Square.

While escorting a diplomatic convoy on September 16, 2007, they opened fire with sniper rifles, machine guns and grenade launchers, allegedly without provocation, in the bustling square, leaving at least 14 civilians dead and at least 17 wounded. The Iraqi government says the toll was higher.

The shooting deepened the resentment of Americans in Iraq four years after US forces toppled dictator Saddam Hussein, and raised questions about the expanded use of armed contract guards by the US government.

Slatten was originally sentenced in 2015 to life in prison as the alleged first person to have opened fire.

The three others were given 30 year prison sentences, although Lamberth at the time of sentencing said that: "It's clear these fine young men just panicked."

An appeals court, accepting later evidence that another guard was the first to fire, overturned Slatten's guilty verdict and ordered a new trial for him alone.

He faced one charge of first degree murder of a single Iraqi, Ahmed Haithem Ahmed al Rubia'y.

A spokesman for the court said federal prosecutors were reviewing the case. They can choose to ask for a new trial, or decide not to pursue it further.