he first bomb landed shortly after sunrise on April 4, 2017, in Khan Shaykhun. Unlike the three that would explode moments later in other parts of the rebel-controlled Syrian town, this one produced little noise and even less physical damage, leaving behind a jagged 5-foot-wide-by-20-inch-deep crater in an otherwise empty road. Minutes earlier, a group of volunteer rescue workers in town had received an ominous alert: Spotters had observed a Syrian Armed Forces bomber taking off from Shayrat airbase 68 miles away, and it was likely carrying a chemical payload. “Guys, tell people to wear masks,” the voice on the other end of the walkie-talkie implored. Most of the town’s 16,000 residents were in bed or getting ready for work when a milky-white cloud began to spread near the bombed-out bakery and grain silos shortly after 6:30 a.m. The first people on the scene arrived to find bodies lying on the ground outside and in homes, with no signs of blunt trauma. Some had bluish lips and were convulsing. Others foamed from the mouth and nose. Nearly all of them had pinpoint pupils. As news of the attack appeared on his computer screen, Stefan Mogl felt a horrible sense of déjà vu. Sitting in his office at Switzerland’s premier national-defense lab, the analytical chemist was all too familiar with the images coming out of Syria that spring morning. Four years earlier, he’d watched hours of similar footage originating from the Damascus suburb of Ghouta, and helped the German magazine Der Spiegel determine that the attack’s victims likely had been exposed to an outlawed nerve agent. He worried that a similar weapon had been used in Khan Shaykhun; a U.N. fact-finding mission would soon confirm the attack had used sarin. Strikes like these are not uncommon in Syria. This past April, the U.N.’s Human Rights Council reported 34 confirmed chemical assaults since the civil war began in 2011 (more than 80 have been reported). Most reputable sources would eventually estimate that up to 100 civilians, including as many as 32 children, died during the Khan Shaykhun attack that day in April 2017—or shortly thereafter.