Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten appeared dressed in suits and ties before a federal court in Washington yesterday, as the second trial in the case entered its fifth week.
Slatten, 32, is charged with the first degree murder of a civilian. He faces life in prison if convicted. Slough, Liberty and Heard are accused of voluntary manslaughter of the 13 other victims.
All four have pleaded not guilty.
Six of the guards had started the shootings.
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FBI expert Douglas Murphy said he travelled to the site of the killings twice to examine the cars involved in the shootings.
As photographs were projected in the courtroom, he spoke of "significant damage" to the 11 vehicles he observed in March 2008 and June 2009.
Murphy pointed to a white KIA riddled with bullet holes, including 29 in the front alone. There was also a Volkswagen that had 13 bullet holes on the driver's side alone.
Asked about the weapons and ammunition used by the four defendants that day, Marine expert Shelby Lasater stressed the grenades used are "designed to penetrate armor and to cause casualties or kill."
"It mushrooms out, it will blow up in a circle and back," he added, noting the grenades could explode across a 540-foot (165-metre) radius.