In October 2002, after Chechen rebels stormed a Moscow theater and trapped more than 800 people for 57 hours, it seemed like it couldn’t get much worse. Then Russian troops released a mysterious gas into the theater. The gas was intended to incapacitate the rebels—which it did—but it also ended up killing more than 120 of the hostages.

That gas contained carfentanil, an opioid 10,000 times more powerful than morphine and 100 times more powerful than fentanyl. Fentanyl has received increased media attention in recent years because of the U.S. opioid crisis, but carfentanil has also been seeping into the American drug market and causing overdose deaths. So yes, carfentanil is a drug that Americans are overdosing on—and it’s also a weapon banned by the Chemical Weapons Convention.

As the then-unknown gas filled the theater, hostages and rebels alike passed out or died immediately. Russian officers dragged everyone out and packed both living and dead hostages onto the same buses and cars, says David Satter, a senior fellow at the Hudson Institute and author of The Less You Know, the Better You Sleep: Russia’s Road to Terror and Dictatorship under Yeltsin and Putin.

“Bodies were piled one on top of another outside the theater entrance, with no attempt to separate the living from the dead,” Satter writes in his book. “Alexander Karpov, a well-known songwriter, died after spending seven hours alive in a bus packed with corpses. In another case, thirty hostages were put in a twelve-seat military microbus, some on the floor. A thirteen-year-old girl was crushed under the bodies and died on the way to a hospital.”

Because Russian officials refused to reveal what was in the gas they’d released, medical professionals didn’t know how to treat the hundreds of victims. They spent several hours testing antidotes before finding that naloxone, a drug used to treat opioid overdoses, could help counter the effects of the gas. By then, more lives had been lost, and the survivors’ health had worsened. Those who lived through the experience continued to suffer from problems that no one knew how to treat, because the gas that’d caused them was still a mystery.

Russia’s rationale for using the gas in the crisis was that officers couldn’t have safely evacuated the hostages unless the rebels were incapacitated. This was because the rebels had announced they’d strung up bombs and some of them were wearing suicide belts. Later, officials discovered that the bombs were dummies, and that most of the suicide belts were fake. In any case, officers “shot all of the terrorists, including those who were unconscious, so that nobody was in a position to dispute their version of events,” Satter says.

Russia has not admitted what was in the gas, and has only acknowledged that it contained fentanyl-related compounds. But in 2012, a group of British scientists analyzed clothing from two survivors and urine from a third survivor. They determined that the gas contained the extremely potent drug carfentanil.

A 2016 AP investigation found that carfentanil is easily available from Chinese dealers, who continue to ship it to the U.S. despite recent collaboration between the countries to limit its export and production. This not only makes it easier for people with opioid addictions to obtain lethal doses, but it also makes it easier for terrorists and authoritarian governments to obtain drugs that many countries recognize as chemical weapons. In a 2017 article for The Cipher Brief, former CIA acting director Michael J. Morell explained how the opioid crisis presents a significant national security threat.

“[C]arfentanil is the perfect terrorist weapon,” he wrote. “It is readily available in large quantities. It comes in several forms—including tablets, powder, and spray. It can be absorbed through the skin or through inhalation. It acts quickly … In short, a single terrorist attack using carfentanil could kill thousands of Americans.”