21 U.S.C. 812 delegates the authority to add, transfer or remove drugs on the five original schedules established by Congress, to the Attorney General and, "pursuant to Executive Order 1172," to the DEA. This delegation of authority was apparently upheld in United States v Davis 564 F.2d 840 (9th Cir 1977), cert. den. 434 U.S. 1015, 54 L.Ed. 760, 98 S.Ct. 733 (1977).
Thanks to Jonathan Moore.