@krieger22 There are a few different bases in the #ChemicalWeaponsConvention and in decisions of the #PolicyMakingOrgans of the #OPCW for attribution work by the OPCW.
The #InvestigationAndIdentificationTeam #IIT spent a chunk of their first report describing their mandate and procedures. That is somewhat specific to the #Syria case and the decision of states parties from summer 2018. https://www.opcw.org/sites/default/files/documents/2020/04/s-1867-2020%28e%29.pdf