The global chemical weapons watchdog on Friday said a nearly two-year investigation had found that at least one Syrian military helicopter had dropped gas cylinders on to residential buildings in the opposition-held Syrian city of Douma in 2018, killing 43 people. The April 7, 2018, attack on the outskirts of Damascus was part of a major military offensive that returned the area to the control of forces under Syrian President Bashar al-Assad after a prolonged Russian-backed siege against the opposition stronghold. A previous investigation by the Organization for the Prohibition of Chemical Weapons (OPCW) had already concluded in March 2019 that a chemical attack had taken place in Douma, but that inquiry had not been mandated to assign blame. The Investigation and Identification Team was established by member states at the Hague-based OPCW in November 2018 to identify perpetrators of chemical attacks in Syria after Russia vetoed the joint UN-OPCW mission. Syria denies using chemical weapons but a previous joint inquiry of the United Nations and the OPCW found that the Syrian government used the nerve agent sarin in an April 2017 attack and has repeatedly used chlorine as a weapon. It blamed ISIS militants for mustard gas use. The latest inquiry identified four alleged perpetrators in one air force unit, but their names were not made public. The findings are based on technical analysis of 70 biological and environmental samples, satellite imagery, 66 witness interviews and ballistic and munitions testing, the OPCW said. "At least one helicopter of the Syrian Tiger Forces' Elite Unit dropped two yellow cylinders containing toxic chlorine gas on two apartment buildings in a civilian-inhabited area in Douma, killing 43 named individuals and affecting dozens more," a summary of the report said. The Tiger Forces are elite Syrian troops generally used in offensive operations in the war. “The world now knows the facts," said OPCW Director-General Ambassador Fernando Arias. "It is up to the international community to take action, at the OPCW and beyond.” The findings follow an investigation conducted between January 2021 and December 2022. The conclusions were "reached on the basis of 'reasonable grounds', which is the standard of proof consistently adopted by international fact-finding bodies and commissions of inquiry," the OPCW said. The OPCW's identification team is deeply opposed by Syria and its military ally, Russia, who say it is illegal. Damascus and Moscow did not cooperate with the latest investigation. Both countries have denied using banned toxic munitions and have instead said the attack in Douma was staged. Several Russian-backed theories about the attack were tested but could not be substantiated, the team found. Those included chlorine cylinders and bodies had been planted at the scene by opposition forces and that the poisonous gas had come from a nearby warehouse used by insurgents. At one location where the largest number of victims was recorded "the cylinder hit the rooftop floor of a three-storey residential building without fully penetrating it, ruptured and rapidly released toxic gas, chlorine, in very high concentrations, which rapidly dispersed within the building killing 43 named individuals", an executive summary said. Weaponizing chlorine is prohibited under the Chemical Weapons Convention, ratified by Syria in 2013. The chemical weapons attack in Douma triggered missile strikes against Syrian government targets by the United States, Britain and France a week later in the biggest Western military action against Damascus during the war which began in 2011.