Weapons photo

The 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.

destroyed site in Khan Shaykhun
The target for the deadly chemical attack in Khan Shaykhun. Hasan/Anadolu Agency/Getty Images

As it turned out, the soft-spoken 52-year-old chemist was a few weeks away from joining the leadership panel of the Joint Investigative Mechanism, a kind of elite international Justice League established in 2015. Formed through a partnership between the United Nations Security Council and the Organization for the Prohibition of Chemical Weapons—the independent intergovernmental body created to oversee compliance with 1997’s Chemical Weapons Convention—the team was tasked with identifying the perpetrators, organizers, and sponsors of chemical attacks in the Syrian conflict. Now that Mogl would be in charge of the technical side of this investigation, he was aware that he and his new position were about to get a lot more attention. “I wouldn’t just be investigating this incident,” he says. “If there’s enough information, I’d be one of the people who would determine responsibility.”

After a nearly two-decade absence from the world stage, banned nerve agents such as ­sarin have re-emerged as modern-day tools for dictators, assassins, and other malefactors. Whether it’s the Russian nerve agent Novichok, used to poison ex-spy Sergei Skripal in the U.K. in March, or the brazen use of VX to murder North Korean despot Kim Jong-Un’s half-brother in broad daylight at the Kuala Lumpur airport in 2017, we are once again living in a world where invisible molecules are regularly being deployed as murder weapons.

From a forensic perspective, it’s easy to see why these illicit agents are attractive. In its purest form, sarin is colorless, tasteless, and odorless, and can kill in minutes. It’s also volatile, meaning it will evaporate from liquid into vapor, and, depending on environmental conditions and the quantity used, murder and maim lots of people before gradually vanishing over the course of days or weeks. While it’s relatively easy to tell if a nerve agent killed or hurt someone, figuring out who deployed it can be notoriously difficult.

young man with oxygen mask
A victim of the Khan Shaykhun assault. Cem Genco/Anadolu Agency/Getty Images

Difficult, but not impossible. As any forensic chemist will tell you, every crime leaves behind traces of molecular evidence. In the same way DNA can provide essential clues about the identity of an individual, toxic substances like sarin and the ingredients used to make it can also provide distinctive signatures. Today, with the help of an increasingly sensitive array of chemistry tools such as gas and liquid chromatography, nuclear magnetic resonance spectroscopy, and mass spectrometry, experts are finding subtle but persistent impurities and other so-called marker compounds in a variety of poisonous weapons. These compounds can be used not only to determine how the toxic chemicals are made and under what conditions, but in combination with other ­evidence, they might help identify the culprit.

Within three weeks of the Khan Shaykhun attack, Mogl was on a plane headed for The Hague, home of the OPCW and soon to be his base of operations for the next five and a half months. He would be working against a deadline; the U.N. had authorized the JIM team’s work only through November 17, 2017. After that, it would be case closed, no matter what the investigation turned up. “We had one critical element, and that was time,” Mogl says. After poring over U.N. documents outlining the parameters of his new investigational powers, Mogl met with colleagues in the Netherlands and formulated a plan for investigating the attack. It would be a crucial set of decisions, particularly since the U.S. had already rendered its own judgment in the form of 59 cruise missiles launched at the Shayrat airbase on April 7.

Enlisting the help of deadly chemi­cals and poisons to kill and maim is a time-honored tradition for humans. Hunters in South Africa were shooting ricin-tipped arrows at least 24,000 years ago. No one knows when we first turned our ­poison-​­making skills to warfare, but until the beginning of the 20th century, military use of this noxious stuff was fairly small scale. Then World War I happened. In 1915, Germany released some 5,700 pressurized cylinders of ­greenish-­yellow ­chlorine gas across 4 miles of the Western Front, changing the world’s perception of chemical weapons. The attack killed more than 5,000 French and Algerian soldiers in a particularly gruesome and painful way: literally corroding the insides of their lungs and throats. Within 10 years, nearly every nation that had deployed poison gas during WWI signed the Geneva Protocol, which prohibited the “use in war of asphyxiating, poisonous or other gases, and of bacteriological methods of warfare.” It didn’t entirely stop the utilization of these lethal agents, but it did lead to a much more comprehensive treaty: the ­Chemical Weapons Convention.

Stefan Mogl
Analytical chemist Stefan Mogl. Courtesy Youtube

Today, 193 countries have signed that agreement, which took effect on April 29, 1997. The treaty bans the development, production, ­acquisition, stockpiling, and transfer of all chemical weapons. Using them on combatants or civilians is also prohibited, and countries must destroy any stockpiles they have when they sign on (or if they have a lot, like the U.S., provide a timeline for destruction).

Twenty-one years in, it’s hard to argue with the results: By the end of 2016, 94 percent of the world’s declared and now-banned stockpiles had been destroyed. That’s 67,753 ­metric tons of poison gone from Earth. But before you start feeling too good about that statistic, note the word “declared.” After an August 21, 2013, sarin attack on Ghouta—still the deadliest chemical-weapon attack in the country—Russia and the U.S. brokered an agreement for the Syrian government to hand over its chemical weapons to the OPCW for destruction. As is evidenced by the continued attacks, Syria clearly neglected to declare some part of its chemical-weapons arsenal.

This is the bleak irony organizations such as the U.N. and OPCW are now grappling with: Despite the elimination of these storehouses of poison (a feat for which the OPCW won the Nobel Prize in 2013), chemical-weapons use is higher today than it’s been in decades.

As head of the ­chemistry division of the Spiez Laboratory in Switzerland, Mogl oversees experts who help implement the CWC, including those who synthesize nerve agents to verify and catalog the routes used to make such weapons. Two months after the Khan Shaykhun attack, and just days into his investigation, Mogl noticed something strange. Scanning through a list of compounds that had turned up in the Khan Shaykhun samples, one in particular leaped out at him: phosphorus hexafluoride, or PF6 in chemistry shorthand. “I just couldn’t place why it was there,” he says. “It wasn’t from the sarin itself, so I thought it must be an impurity that was ­either carried forward during the synthesis process or formed at some other point.”

truck in house rubble
A local hospital hit by government bombs. Mohammed Karkas/Anadolu Agency/Getty Images

Six feet tall, with a slim, athletic build, Mogl is a little like a Swiss Joe Friday—if Joe Friday was also really good with the periodic table. His no-nonsense, “only the facts” approach to scientific investigations is highly valued within the OPCW, given the political posturing, ­finger-­pointing, and allegations of bias that often follow its inquiries. Mogl also has a gift for explaining complicated science to diplomats, a skill that’s made him a popular figure both within the organization and among the scientists who work in affiliated labs. There are few positions he hasn’t held at the group over the past two decades. In 1997, Mogl was a part of the first wave of inspectors sent out to ensure that countries that had signed on to the CWC were actually destroying their declared stockpiles. Three years later, he was running the organization’s main laboratory in Rijswijk, Netherlands. There, he trained analytical chemist inspectors and managed the proficiency tests that international labs had to pass to become OPCW-certified.

Mogl suspected the OPCW had kept samples from Syria’s declared 581-metric-ton stockpile of a key sarin ingredient before overseeing its destruction in 2004. “I figured an analysis of those precursor samples would either show there was no link between Khan Shaykhun and the stockpile, or maybe I can find more about these marker chemicals,” he says.

That summer, Mogl devised a forensics plan to investigate the origin of the PF6 in the Khan Shaykhun samples. He wanted to know whether the samples from the 2014 Syrian stockpile contained the same compound. A lab analysis confirmed this was the case—for every single sample. He also had to determine how the impurity ended up in those Syrian chemicals and, more important, when. Sarin is typically stored and delivered in a munition as two separate ingredients: isopropyl alcohol—a purified version of what you buy at the drugstore to clean cuts—and methyl­phosphonic difluoride, or DF—the material Syria had turned over to the OPCW. Chemists call these precursors. Once made, sarin usually lasts only a few weeks or months. This binary method keeps two relatively stable precursor ingredients separate until right before they’re used. In essence, the nerve agent is stored and delivered in its penultimate state. Figuring out how the PF6 got into Syria’s stockpile of DF could help reveal if the marker could be used to flag their weapons. But it would ­require making some batches from scratch.

United Nations chemical warfare discussion
U.N. ambassador Nikki Haley denouncing Syria’s regime. U.N. Photo/Rick Bajornas/SIPA/Newscom

Because DF is virtually impossible to obtain legally, most manufacturers make it from another chemical: methylphosphonic dichloride, or DC. It’s slightly less dangerous and easier to get your hands on. Swap the ­chlorines of this white crystalline solid with fluorines, and you’ve got yourself some DF. This can be done a number of ways. Most chemists would use sodium fluoride (the cavity-fighting stuff found in your toothpaste); it’s safe and works perfectly as a fluorinating agent. Another option—more common in industrial-­scale ­operations—is hydrogen fluoride, or HF. A much more aggressive and dangerous chemical, HF is harder to work with but yields more DF. Using it would suggest a high degree of competence and sophistication on the part of the mixer making the DF. It would also suggest the manufacturer was making tons of it.

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Weapons photo

When Mogl’s team had independent labs use hydrogen fluoride to turn DC into DF, PF6 always appeared. When they used other fluorinators, it never appeared. Armed with this new information and other impurities data, Mogl grew increasingly confident that the sarin used in Khan Shaykhun must have been made with some of the same precursor that Syrian authorities had handed over between October 2013 and June 2014.

With a damning pile of evidence in their hands, Mogl and his team were now running out of time. On October 18, 2017, with the chemistry portion of his report yet to be written, Mogl flew to New York City (where the JIM team’s political branch was based), checked in to a hotel, and got to work. “I had a clear picture of what the chemistry section should contain,” he says, “but it also needed to be readable for non-experts yet still technically precise.” After several drafts and some editorial guidance from multiple team members, Mogl (and U.N. lawyers) felt they had crafted a chemically compelling argument.

United Kingdom military cleaning up poison
U.K. military cleaning the site of last March’s spy poisoning (above); U.N. ­ambassador Nikki Haley denouncing Syria’s regime. Matt Cardy/Getty Images

Eight days later, the JIM team submitted a 33-page report to the U.N. Security Council. Within 12 minutes of being issued, someone leaked it. The conspiracy theories flooded in: false-flag operations carried out by terrorist groups, forged evidence by pro-American groups, elaborate hoaxes by Syrian rebels. “We knew what we were getting into,” Mogl says of the attempts to discredit the report and the people conducting it. “As far as I know, no one has questioned the chemistry.”

Twelve days after that, at an official U.N. Security Council briefing, the head of JIM, Edmond Mulet, summarized its findings to the world: “The sarin used in Khan Shaykhun was very likely to have been made from the same precursor that came from the original stockpile of the Syrian Arab Republic,” he told members. The final report summarized the results of assessing video recordings, photos, and satellite images; interviews with eyewitnesses; and analyses from experts on explosives and smoke plumes. But it was Mogl’s sleuthing that ultimately helped make one of the strongest scientific cases for responsibility, laying the grisly deaths at the feet of Syrian President Bashar al-Assad’s regime. “We knew this report was going to be read by the entire world, that every word was going to be scrutinized,” Mogl says, “but at the end, I was totally ­confident about the attribution.”

Related: It’s not just Syria—chemical weapons still pose a global threat

Mogl, now back at Spiez, is less confident in the world’s ability to curtail the use of chemical weapons. In the year following the JIM report, not much has changed in Syria. Russia, a member of the U.N. Security ­Council, refused to renew the mandate of the investigative commission, which disbanded when its time was up. And while treaty members voted this past June to ­double down on efforts to identify any state or nonstate actor that decides to use chemical weapons, what that will look like ­remains ­undecided. Since Mogl and his colleagues ­r­eleased their report, there have been at least four more chemical attacks in Syria, including one in the city of Douma in April, when dozens of people were reportedly killed by chlorine bombs. No investigation into accountability is currently underway. But Mogl and an international crew of chemical sleuths are standing by should the world call on them again—ready to find traces of a smoking chemical gun.