WASHINGTON (AFP) Two former Blackwater security guards went on trial Tuesday accused of the murder of two Afghan citizens in a 2009 shooting. Christopher Drotleff, 29, and Justin Cannon, 28, pleaded not guilty on August 17 to a total of 13 charges connected to the incident, which they say was legitimate self-defence. If convicted, they face life in prison. The defendants are there and jury selection is in progress, Peter Carr, a spokesman for the Virginia state attorney, told AFP. Cannon and Drotleff were employed by Paravant, an affiliate of Xe, formerly Blackwater, which gained notoriety in Iraq after guards protecting a convoy opened fire in a busy Baghdad square in September 2007, killing as many as 17 civilians. This case involves a shooting on May 5, 2009 at the scene of a traffic accident in Kabul in which the two guards are alleged to have opened fire with an AK-47 and a 9mm pistol, killing two people and wounding one. The guards had left their base without permission and were escorting a vehicle carrying three interpreters to a taxi stand when there was an accident. The defence maintains the accident was caused by a third vehicle and that the guards opened fire to protect themselves. But the prosecution alleges that the third car had stopped to render assistance, but tried to leave the scene after the Paravant guards, who had been drinking heavily earlier in the day, became combative. It alleges that Cannon and Drotleff then opened fire on the departing vehicle, killing a passenger and a pedestrian bystander. The vehicles driver was wounded but survived. The Afghan government announced in August that it was giving security firms operating in Afghanistan four months to dissolve, a measure that could create huge problems military and other international entities that depend on the estimated 40,000 employees of private security contractors.