Two ex-employees of a security company owned by the US firm formerly known as Blackwater have pleaded not guilty to charges of killing two Afghans in 2009.
Justin Cannon and Christopher Drotleff allegedly opened fire at the scene of a road accident in Kabul.
They worked for Paravent, part of Xe, formerly Blackwater, which was blamed for killing civilians in Iraq in 2007.
Afghanistan's President Hamid Karzai has said private security firms must end operations there in four months.
The ultimatum is expected to create big problems for Nato and other international bodies which rely on tens of thousands of private security employees.
The legal team for the two men maintains the guards opened fire at the scene of the accident in self defence. The trial is due to start on 14 September in Norfolk, Virginia.