The State of GA's charges of murder are superior to the fed civil rights charges. If the feds want to come in later, they can do that. GA DAs don't need permission from the feds to bring murder charges. I doubt that any of the victim's family are members of the GA bar.
Reading the last part of that article and that they pled guilty to manslaughter
seems like a deal may have been cut for them to roll over on other narcotics officers.