Former Blackwater Worldwide guard Nicholas Slatten leaves federal court in Washington on June 11, 2014. (Cliff Owen/AP)

The U.S. government will retry a former Blackwater Worldwide security guard after a federal appeals court threw out his first-degree murder conviction in shootings that killed 14 unarmed Iraqi civilians in Baghdad's Nisour Square in 2007, prosecutors told a federal judge Tuesday.

Assistant U.S. Attorney Patrick Martin of the District said the government will seek a retrial of Nicholas A. Slatten, 33, of Sparta, Tenn., as early as May, anticipating a six-week case with 50 witnesses, including about 15 from Iraq.

U.S. District Judge Royce C. Lamberth scheduled a Dec. 14 hearing to set a trial date and decide whether to conditionally release Slatten from federal prison in Sumterville, Fla., where he had been serving a mandatory life sentence.

A three-judge panel of the U.S. Court of Appeals for the D.C. Circuit on Aug. 4 tossed out Slatten's 2014 conviction on one count of murder, saying the trial court erred in not allowing him to be tried separately from three co-defendants, even though one of them said he, not Slatten, fired the shots that killed the first civilian victim, leading a team of U.S. security contractors to open fire indiscriminately on vehicles and pedestrians.

The court denied a government request for rehearing before the full court on Nov. 6.