Blackwater Worldwide security guard Nick Slatten, center, leaves the federal courthouse after being arraigned with 4 fellow Blackwater guards, Jan. 6, 2009 in Washington.
Blackwater Worldwide security guard Nick Slatten, center, leaves the federal courthouse after being arraigned with 4 fellow Blackwater guards, Jan. 6, 2009 in Washington.

The retrial of a Blackwater security guard alleged to have participated in a 2007 massacre of unarmed Iraqi civilians ended Wednesday in a hung jury.

U.S. District Judge Royce C. Lamberth declared a mistrial after the jury said it remained deadlocked after more than two weeks of deliberations in the case of Nicholas A. Slatten.

Slatten was accused of firing the first shots of a one-sided firefight on Sept. 16, 2007, in Baghdad's Nisour Square. Armed Blackwater personnel fired machine guns and threw grenades into traffic, killing or injuring 31 people.

Since that time, the U.S. Justice Department has been trying to hold the Blackwater employees involved in the incident responsible.

In 2014, Slatten was convicted of first-degree murder and sentenced to life in prison. Last August, a federal appeals court threw out that conviction and ordered a new trial.