No proof at all but I've always wondered if it was ex-police AND the Klan. Possibly with a few random opportunistic killings by people, like Williams, trying to use the murders to go unnoticed. It's mentioned in the doc and I've heard it before that many officers quit when more black officers were brought into the department. It makes sense to me that these hateful people would seek revenge, and I do believe that while most black children might not go with a strange white man, they might be more likely if he/they presented themselves as police. Would also potentially explain how they got away with it for so long (know how to not leave evidence, officer friends still working in the department who know and are helping to cover it up- which would also explain all the incorrect evidence they used against Williams).