OPCW identifies ISIL as perpetrators of 2015 chemical attack in Marea, Syria

IIT report concludes ISIL held exclusive means, motives, and capabilities to deploy sulfur mustard in 2015 attack

22 February 2024

THE HAGUE, Netherlands—22 February 2024—The Organisation for the Prohibition of Chemical Weapons (OPCW) Investigation and Identification Team (IIT)’s fourth report concludes that there are reasonable grounds to believe that units of the Islamic State in Iraq and the Levant (ISIL) were the perpetrators of the chemical weapons attack on 1 September 2015 in Marea, Syria. The IIT’s comprehensive investigation was conducted from January 2023 to February 2024.

Key findings

1. Sulfur mustard attack by ISIL:

  • The IIT concludes that there are reasonable grounds to believe that on 1 September 2015, between 09:00 and 12:00 (UTC+3), during sustained attacks aimed at capturing the town of Marea, units of the Islamic State in Iraq and the Levant (ISIL) deployed sulfur mustard.
  • The chemical agent was delivered using one or more artillery guns.

2. Impact locations and remnants:

  • The IIT identified several impact locations across Marea, with no discernible targeting pattern.
  • All remnants and munitions observed at these sites were conventional artillery projectiles, specifically of a 122-mm calibre, modified to disperse a liquid payload.
  • Upon impact, at least six projectiles leaked a black, viscous substance with a distinct “pungent” and “garlic-like” smell.
  • Eleven named individuals who came into contact with the liquid substance experienced symptoms consistent with exposure to sulfur mustard.

3. ISIL’s exclusive and dedicated capability in this incident:

  • The IIT established that the chemical payload was deployed by artillery from areas under ISIL control.
  • No other entity possessed the means, motives, and capabilities to deploy sulfur mustard as part of an attack in Marea on 1 September, 2015.
  • The IIT was able to reconstruct the organisational structure and chain of command that led to the use of chemical weapons by the Islamic State in Marea on 1 September 2015.
  • Strategic military operations such as the attack carried out in Marea, involving the large-scale deployment of chemical weapons, would only have occurred pursuant to direct orders from ISIL’s Executive Branch, i.e., Delegated Committee, operating directly under ISIL’s so-called “Caliph”, according to IIT conclusions.
  • In the course of its investigation, the IIT was able to link additional organisational structures and individuals to the use and deployment of chemical weapons by the Islamic State, including ISIL’s Diwan Al-Jund (Department of Soldiery) and the Committee for Military Development and Manufacturing, identifying a total of four named individuals as perpetrators. Two further ISIL members were identified as the primary drivers of ISIL’s chemical weapons programme.

4. Degree of certainty:

  • The IIT reached its conclusions based on the standard of proof known as “reasonable grounds”, consistently adopted by international fact-finding bodies and commissions of inquiry.
  • The assessment included information from the Fact-Finding Mission (FFM), States Parties, interviews conducted by the IIT, and analyses of samples, computer modelling, satellite imagery, front-line maps, authenticated videos and photos, and other relevant data.

Upon releasing the report, OPCW Director-General Ambassador Fernando Arias said: “With the fourth report by the Investigation and Identification Team (IIT), the Secretariat of the OPCW has once again delivered on the mandate it has received to identify perpetrators of chemical weapon use in Syria. In this report, the IIT concluded that ISIL used sulfur mustard in Marea on 1 September 2015. This independent investigation has been conducted based on a sound scientific methodology and on the combination, consistency, and corroboration of all of the information gathered and analysed, in line with the highest international standards.

Once again, the OPCW Secretariat has demonstrated that it is able to identify perpetrators of the use of chemical weapons, States and non-State actors alike. This is a stark reminder to the international community that non-State actors like ISIL have developed the capacity and the will to use chemical weapons. This emphasises the OPCW’s pivotal role and expertise in addressing such threats. The facts are now known – it is up to the international community to take action, at the OPCW and beyond.”


The mandate of the IIT is to identify the perpetrators of specific instances of chemical weapons use in the Syrian Arab Republic. It is responsible for investigating only those instances in which the Fact Finding Mission (FFM) has determined that use or likely use of chemical weapons in Syria has occurred, as well as cases for which the now expired OPCW-United Nations Joint Investigative Mechanism (JIM) did not issue a report.  

The IIT is tasked with fact-finding and is not a prosecutorial or judicial entity. It is not responsible to determine criminal responsibility of individuals, organisations or States. Furthermore, the IIT does not make recommendations for future action. These issues pertain to the policy-making organs of the OPCW (i.e., the Conference of States PartiesExecutive Council) and other relevant bodies. According to the decision of the Conference of States Parties, the report is shared with the United Nations Secretary-General.  

As the implementing body for the Chemical Weapons Convention, the OPCW, with its 193 Member States, oversees the global endeavour to permanently eliminate chemical weapons. Since the Convention’s entry into force in 1997, it is the most successful disarmament treaty eliminating an entire class of weapons of mass destruction.

In 2023, the OPCW verified that all chemical weapons stockpiles declared by the 193 States Parties to the Chemical Weapons Convention since 1997 —totalling 72,304 metric tonnes of chemical agents— have been irreversibly destroyed under the OPCW’s strict verification regime.

For its extensive efforts in eliminating chemical weapons, the OPCW received the 2013 Nobel Peace Prize.

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