Congress set the initial scheduling of some drugs, and set a process for updating the schedules within the executive branch going forward, with the DEA (in statute, the Attorney-General, but in practice within Justice its the DEA doing the main work) playing the key role (HHS in statute -- via the FDA in practice-- plays a mandatory role in advance of the decision, which is controlling in the case of a currently non-controlled drug that HHS recommends not controlling, but advisory in, I believe, all other cases.)