In a statement made the day after the September 2007 shooting, Slough told US State Department agents he opened fire on a car because "the driver looked directly at me and kept moving toward our motorcade. Fearing for my life and the lives of my teammates, I engaged the driver and stopped the threat."
U.S. investigators say the driver of the car was an unarmed civilian.
Heard, in his statement to agents, said he "heard gunshots" and "engaged the car with approximately 20 to 30 rounds from my M4 rifle."
Heard's attorney, David Schertler, said they strongly disagree with the decision to bring charges against Heard, which he characterized as "wrong and unjust."
Schertler said: "On September 16, 2007, at Nisour Square, Blackwater security guards were defending themselves and their comrades who were being shot at and receiving fire from Iraqis they believed to be enemy insurgents, in a place where the enemy has made a major city, Baghdad, the battlefield. Because of that and because of the deceptive tactics used by these insurgents against Americans, civilian casualties tragically occur. We have been and remain determined and prepared to fight these charges and we are confident that Dustin Heard will be vindicated."
Lawyers for the other Blackwater guards declined to comment. Blackwater says it has cooperated with the investigation and officials say no criminal charges are expected against the company itself.