DHS, DOJ, and Customs and Border Patrol Policies on Use of Deadly Force and Moving VehiclesFleeing Subjects: Deadly force shall not be used solely to prevent the escape of a fleeing subject. However, deadly force is authorized to prevent the escape of a fleeing subject where the LEO has a reasonable belief that the subject poses a significant threat of death or serious physical harm to the LEO or others and such force is necessary to prevent escape.
Not that laws matter, but several laws say that DHS employees cannot kill people driving away from them, which is common sense.