Four American guards for the Blackwater Worldwide security firm facing manslaughter charges for allegedly slaying Iraqi civilians in 2007 have asked the U.S. Supreme Court to dismiss their indictment.

The guards for the private security contractor, since renamed Xe, were indicted on voluntary manslaughter charges and weapons violations for a shooting in Baghdad's Nisur Square as part of an effort to get a diplomat safely to the Green Zone.

The shooting--a dispute over who shot first and when is the crux of the underlying criminal case--left 14 Iraqi civilians dead and 20 wounded.

A U.S. District Court judge in Washington, D.C., dismissed the indictment in 2009 because evidence shown to a grand jury was tainted due to the indicted employees' written statements appearing in the media. The guards' statements were also provided under a guarantee that they could not be used against them in a criminal proceeding.

 A three-judge appellate panel in 2009 overturned the decision and reinstated the case.

The four Blackwater guards--Paul A. Slough, Evan S. Liberty, Dustin L. Heard and Donald W. Ball--petitioned the Supreme Court, which docketed the filing last week, to reinstate the 2009 district court ruling that dismissed the indictment. A fifth guard involved in the Nisur Square shooting had charges against him dropped. A sixth pled guilty in 2008 to charges of voluntary and attempted manslaughter.

Prosecutors accused the guards, whose team had blocked off traffic while escorting the diplomat, of opening fire with automatic weapons and grenade launchers on unarmed civilians and occupied cars. The guards have said that they were defending themselves from a threat.

The Supreme Court petition was filed days before President Barack Obama on Friday announced that American troops will leave Iraq at the end of the year.