THE HAGUE, Netherlands—An investigation by the global chemical weapons watchdog found “reasonable grounds to believe” that a Syrian air force military helicopter dropped a chlorine cylinder on a Syrian town in 2018, sickening 12 people, the Organization for the Prohibition of Chemical Weapons said Monday. It is the second time that the OPCW’s Investigation and Identification Team has concluded that Syrian government armed forces likely were responsible for a gas attack. Last year, the team also found reasonable grounds to believe that the Syrian Arab Air Force was responsible for attacks using chlorine and the nerve agent sarin in March 2017 in the town of Latamneh. Syria has repeatedly been accused of using chemical weapons during the country’s grinding civil war. The government of President Bashar Assad denies the claims. In the latest report, the OPCW investigation team said it found evidence that a military helicopter belonging to the Tiger …