Timeline of investigations into Syria's chemical weapons

FILE PHOTO: A youth who survived from what activists say is a gas attack cries as he takes shelter inside a mosque in the Duma neighbourhood of Damascus, Syria August 21, 2013. REUTERS/Mohamed Abdullah/File Photo

(Reuters) - 2013 - U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov reach an agreement on the elimination of Syria’s chemical weapons.

- U.N. Secretary General Ban Ki-moon establishes United Nations Mission to Investigate Allegations of the Use of Chemical Weapons in the Syrian Arab Republic. Headed by Swedish scientist Ake Sellstrom and comprised of experts from the World Health Organization and the Organisation for the Prohibition of Chemical Weapons (OPCW), it is tasked with looking into possible use of chemical weapons following reports of an attack in the northern town of Khan al-Assal. It confirms use of sarin in the Aug. 21 attack in the Damascus suburb Ghouta that killed hundreds.

2014 - OPCW Declaration Assessment Team begins work to resolve gaps and inconsistencies in Syria’s declaration to the OPCW, which was supposed to include all aspects of its chemical weapons program. By July 2016, after visiting Syria 18 times to inspect sites and meet with Syrian officials, it can “not fully verify that Syria had submitted a declaration that could be considered accurate and complete.”

- OPCW establishes a Fact Finding Mission in response to persistent allegations of chemical weapon attacks in Syria, “to establish facts surrounding allegations of the use of toxic chemicals, reportedly chlorine, for hostile purposes in the Syrian Arab Republic.” The Fact Finding Mission concludes the use of chlorine was systematic and widespread. It is not tasked with assigning blame.

2015 - United Nations-OPCW Joint Investigative Mechanism in Syria is established to identify individuals or entities behind chemical weapons attacks.

2016 - Joint Investigative Mechanism concludes that Syrian government forces used chlorine as a chemical weapon in three cases and that Islamic State militants used sulphur mustard.

- OPCW executive council adopts decision condemning Syrian government and Islamic State for chemical weapons use after a vote that split the body and signaled an end to U.S.-Russia cooperation.

2017 - OPCW Fact Finding Mission concludes that sarin was used in an April 4 attack in the Khan Sheikhoun area of northern Syria, the most deadly use of the nerve agent in three years. It does not assign blame.

Editing by Janet McBride and Richard Woods