Diplomats and scientists announced that the chemical weapons stockpile of the Syrian regime was linked to the largest sarin nerve gas attack in the war. Laboratory tests confirmed the findings, supporting claims that the regime of Bashar Assad was behind the August 21, 2013 attack in the Damascus suburb of Ghouta, said an exclusive Reuters report. Laboratories working for the Organization for the Prohibition of Chemical Weapons compared samples taken by a UN mission in Ghouta after the attack, when hundreds of civilians died of sarin gas poisoning, to chemicals handed over by Damascus for destruction in 2014. The tests found “markers” in samples taken at Ghouta and at the sites of two other nerve agent attacks, in the towns of Khan Sheikhoun in Idlib governorate on April 4, 2017 and Khan al-Assal, Aleppo, in March 2013, two people involved in the process said. “We compared Khan Sheikhoun, Khan al-Assal, Ghouta,” said one source who asked not to be named because of the sensitivity of the findings. “There were signatures in all three of them that matched.” The same test results were the basis for a report by the OPCW-United Nations Joint Investigative Mechanism in October which said the regime was responsible for the Khan Sheikhoun attack, which killed dozens. The findings on Ghouta, whose details were confirmed to Reuters by two separate diplomatic sources, were not released in the October report to the UN Security Council because they were not part of the team’s mandate. They will nonetheless bolster claims by the United States, Britain and other Western powers that Assad’s regime still possesses and uses banned munitions in violation of several Security Council resolutions and the Chemical Weapons Convention. The OPCW declined to comment. Syria has repeatedly denied using chemical weapons in the conflict now in its seventh year and has blamed the chemical attacks in the rebel-held territory of Ghouta on the insurgents themselves. Russia has also denied that Syrian regime forces have carried out chemical attacks and has questioned the reliability of the OCPW inquiries. Officials in Moscow have said the rebels staged the attacks to discredit the Assad regime and whip up international condemnation. Under a US-Russian deal after the Ghouta attack in 2013, Damascus joined the OPCW and agreed to permanently eliminate its chemical weapons program, including destroying a 1,300-ton stockpile of industrial precursors that has now been linked to the Ghouta attack. But inspectors have found proof of an ongoing chemical weapons program in Syria, including the systematic use of chlorine barrel bombs and sarin, which they say was ordered at the highest levels of regime. “A match of samples from the 2013 Ghouta attacks to tests of chemicals in the Syrian stockpile is the equivalent of DNA evidence: definitive proof,” said Amy Smithson, a US nonproliferation expert. She and other sources familiar with the matter said it would have been virtually impossible for the rebels to carry out a coordinated, large-scale strike with poisonous munitions, even if they had been able to steal the chemicals from the regime’s stockpile. “I don’t think there is a cat in hell’s chance that rebels or ISIS were responsible for the Ghouta attack,” said Hamish de Bretton-Gordon, an independent specialist in biological and chemical weapons. The UN-OPCW inquiry, which was disbanded in November after being blocked by Syria’s ally Russia at the UN Security Council, also found that ISIS had used the less toxic blistering agent sulfur mustard gas on a small scale in Syria. The Ghouta attack, by comparison, was textbook chemical warfare, Smithson and de Bretton-Gordon said, perfectly executed by forces trained to handle sarin, a toxin which is more difficult to use because it must be mixed just before delivery.