The Organization for the Prohibition of Chemical Weapons (OPCW)-led Investigation and Identification Team (IIT) has concluded, based on an extensive inquiry examining munitions remnants, samples, testimony, and documentation, that there are reasonable grounds to determine that the Islamic State in Iraq and the Levant (ISIL) deployed sulphur mustard, a banned chemical warfare agent, during strikes targeting Marea, Syria on 1 September 2015, via modified artillery projectiles. The IIT’s technical analyses linked the sulphur mustard’s characteristics to production methods consistent with ISIL’s chemical weapons programme. This investigation, directly implicating ISIL in a chemical attack for the first time, underscores the threat posed by terrorist groups developing chemical munitions, prompting calls for accountability and prevention efforts.