the source is the Miami Herald. They claim to have seen FBI documents referencing a bunch of independent reports from different parts of the prison. A guard on the gate where the dumpsters are notified the FBI that he was seeing very high volumes of shredded paper come out. An inmate tasked with trash transport told one of his guards that BOP investigators were having him transport bags of shredded documents, which he felt was counter to an investigation. There were more beyond those two.