The Organization for the Prohibition of Chemical Weapons says it has found “incontrovertible” evidence that the chemical attack in Syria this month included sarin nerve gas. Eighty-nine people, mostly civilians and many of them children, were killed in the April 4 attack on the town of Khan Sheikhoun; President Trump responded by firing missiles at the airfield from which the attack was launched. The OPCW, the international arbiter of the 1997 Chemical Weapons Convention, took biomedical samples from 10 victims and says it determined that they were exposed to sarin “or a sarin-like substance.” “While further details of the laboratory analyses will follow, the analytical results already obtained are incontrovertible,” said Ahmet Uzumcu, the organization’s director general, said in a statement. He added that samples were taken during autopsies and from other victims while they underwent medical treatment. Scientists in the U.K. have said they also found evidence of sarin gas from samples taken at the site.