On March 4th, former Russian double-agent Sergei Skripal and his daughter Yulia were found unconscious in Salisbury, England. They were the victims of an apparent poisoning. The poison was identified this week as a nerve agent called Novichok, part of a group of chemical weapons said to be extremely potent that we know very little about.

Even before the lab results were out, it looked like the Skripals were the victims of a nerve agent: Yulia Skripal was unconscious, seizing, vomiting, and had lost control of her bodily functions, the BBC reports; Sergei Skripal had gone rigid and immobile, according to CBS News. (Neither has died, but both are hospitalized in critical condition.) Since the Novichok nerve agents were developed in the Soviet Union at the end of the Cold War, British Prime Minister Theresa May blamed Russia for the poisoning. Though Moscow denies any involvement, the attack stokes Cold War-era tensions — heightened by the fact that this deadly chemical weapon is so mysterious.

“The U.S.S.R. is the only country to have developed and produced these [Novichok] agents,” Jean Pascal Zanders, formerly a senior research fellow at the European Union Institute for Security Studies, told Mark Peplow at Chemical & Engineering News. “It’s almost as though the Russians are sending a message to the West that they can reach anywhere, whenever they like.”

The attack stokes Cold War-era tensions

The Novichok agents were created under a clandestine program that continued despite international negotiations for a chemical weapons ban, according to former Soviet scientist and defector Vil Mirzayanov. That secrecy is why we still don’t know their exact chemical makeup. What we do know is that Novichok, Russian for “newcomer,” is actually a collection of chemical weapons that only become lethal after two, somewhat less deadly ingredients are mixed. These so-called binary nerve agents are thought to be safer to store, Mirzayanov wrote in 1995. But they’re also easier to hide from inspectors, especially if those two ingredients could masquerade as components of fertilizer or pesticides.

We also don’t know how exactly Novichok kills. Other nerve agents — like the sarin gas used against civilians in Syria or the VX that killed Kim-Jong Nam, the half-brother of North Korea’s leader Kim Jong-un — get into the body through breathing, eating, or through the skin. Once inside, they block an enzyme that’s key for healthy signaling between nerves and muscles. That leads to symptoms like drooling, seizures, and paralysis. “You’re tearing, you have a runny nose, you have fluid in your lungs, you have lots of diarrhea, you’re sweating, and these agents also slow your heart rate down,” says Peter Chai, a medical toxicologist at Brigham and Women’s Hospital in Boston. “You basically asphyxiate in your own secretions. So, it’s a horrible way to die.”

“It’s a horrible way to die.”

At the right doses, nerve agents can kill within five to 15 minutes, says chemical weapons expert Mark Bishop at the Middlebury Institute of International Studies at Monterey. But the Novichok agents are thought to be even more dangerous and deadly; Mirzayanov claims that Novichok-5, for example, can be five to eight times more potent than VX. So the fact that the Skripals are still alive means that “it must have been low dose, or impure, or not administered in a really efficient way,” Bishop tells The Verge. “Because it doesn’t take very much of a nerve agent to be fatal.”

Treating Novichok poisoning is also “practically impossible,” according to the Handbook of Toxicology of Chemical Warfare Agents. For other nerve agents, the treatment is usually diazepam or Valium to stave off seizures, and atropine, which helps dry up the secretions that could choke or drown a nerve gas victim, Chai tells The Verge. That buys a little time for another drug called pralidoxime, or 2-PAM, to prevent the nerve agent from permanently shutting off that key enzyme. But Novichok agents might have more ways of harming people, according to the Handbook: “Consequently, conventional nerve agent antidotes may not work.”

“It doesn’t take very much of a nerve agent to be fatal.”

It’s surprising to see Novichok surface in 2018, not least because Russia was supposed to have destroyed its 39,967 metric tons of chemical weapons by September 2017, according to the international organization that oversees the chemical weapons ban. The British prime minister has demanded that Moscow release information about the Novichok program, so it’s possible that we could soon learn more about these nerve agents.

Bishop, however, says not to bet on it: “The Russians are really good at keeping secrets.”