Early last week, in an airport in Kuala Lumpur, two women approached Kim Jong Nam—estranged half-brother of North Korean dictator Kim Jong-un—from behind. They swiped what the victim described to nearby customer service agents as a “wet cloth” across his face, and fled. Shortly after, he was dead.

Now, Malaysian authorities say they’ve identified the substance that took Jong Nam’s life: VX, a nerve agent that the United Nations classifies as a weapon of mass destruction. And while it’s not an entirely uncommon substance—or particularly difficult to produce—its apparent use marks a troubling break from international norms. And if officials manage to link it back to North Korea, it could have serious consequences.

Special VX

If you’re already familiar with VX agent, it’s likely because of seminal 90s action flick The Rock, in which a disgruntled Ed Harris brings over a dozen VX-laden warheads along with him to seize Alcatraz.

VX doesn’t work quite the way The Rock depicts it. Specifically, contact with it doesn’t cause human skin to bubble and sear. But it plays havoc with the human nervous system. Like other nerve agents, VX interferes with the signals that pass between your brain and your muscles. “If you have a nerve impulse that tells a muscle to contract, you have to turn off the impulse. Otherwise the muscle will stay contracted,” says Matthew Meselson, a geneticist at Harvard and member of the Center for Arms Control and Non-Proliferation national advisory board. “The one that primarily kills is a spasm of the diaphragm, so you can’t breathe. You die of asphyxiation.”

VX can work through skin contact or respiration, and while it’s part of a broader class of nerve agents that all accomplish roughly the same effect, experts consider it to be especially dangerous, even among banned substances. “It’s heavier than other nerve agents, so it settles on an environment and can be persistent on the ground. If it was used in larger quantities, it could make an area non-usable,” says Tom Inglesby, director of the Johns Hopkins University Center for Health Security.

As the Kim Jong-nam incident showed, though, smaller quantities are also dangerous. “Even a tiny drop is lethal,” Inglesby says.

And while an antidote exists—atropine, which unlocks the muscles that VX causes to seize up—the nerve agent works so quickly that it’s no use unless there’s a hypodermic needle on scene.

So dangerous is the stuff, in fact, that all but a handful of countries agreed to destroy whatever stockpiles they had of VX as part of the Chemical Weapons Convention of 1993. One of the handful of holdouts: North Korea.

The Red Line

In 1995, Japan’s Aum Shinrikyo cult turned the nerve agent on a small number of its members, whom leaders believed to be police informants. On a larger scale, VX was one of the chemical weapons deployed in the Iran-Iraq war. The Kim Jong Nam case, though, would be the first VX assassination on record, and the first time chemical weapons were used to that end since a ricin pellet—fired from an umbrella gun—took Bulgarian dissident Georgi Markov’s life in 1978.

“That this particular chemical weapon would be used in a political assassination in a third country is very alarming. It’s a red line,” says Ingelsby. “It should be considered a new threshold that’s been crossed in terms of unconventional weapons.”

Those norms matter. After decades without any nation deploying chemical weapons, Syria used sarin and chlorine gas. If a nation-state such as North Korea uses VX once, they or other actors may well do it again.

'It should be considered a new threshold that's been crossed in terms of unconventional weapons.' Dr. Tom Inglesby, Johns Hopkins University Center for Health Security

That’s all conditional for a reason. While North Korea maintains a VX stockpile, and Kim Jong Un may well have considered his half-brother a threat to his rule, there’s no direct link between the VX airport incident and the hermit kingdom. And there may well never be, at least from the weapon of choice.

“It’s not very hard to produce, so it’s doubtful that the specific use can be chemical-traced back to North Korea,” says Scott Sigmund Gartner, director of the Penn State School of International Affairs. Any decent organic chemist can make the stuff.

Meselson also says that it may not have been VX at all; if it was, it’s remarkable that the two women survived the attack as well.

All of which underscores how critical the next several days of investigation will be. If it turns out to be a random or untraceable act, it may at least prove to be an isolated incident. Should a direct link to North Korea exist, the world will find itself in potentially dangerous, uncharted waters.

“The political reaction should be very strong internationally, once all the facts are in,” says Ingelsby. “Responsible countries around the world should make it very clear that this kind of behavior is unacceptable.”

Unfortunately, that’s the thing about red lines. Once you cross them, there’s no going back.