For nearly three decades, since a Soviet whistleblower told the world of its existence, the nerve agent Novichok has scared American weapons experts. The Pentagon sent teams to destroy abandoned laboratories that once produced the chemical, believed to be orders of magnitude more lethal than sarin or VX.

There was no sign of it ever being used. Until last week.

Now, Britons are taking in the disquieting information that a Novichok nerve agent, a weapon invented for use against NATO troops, was released in the quiet town of Salisbury, its target a former Russian spy named Sergei Skripal. Skripal and his daughter Yulia remain hospitalized.

Although U.S. laboratories stopped producing nerve agents around 1970, after the production of so-called third-generation nerve agents like sarin and VX, Soviet scientists continued their work for two decades, producing a fourth generation. The Novichok nerve agents came in solid form, like a powder or thick paste, and would not register on the chemical detector paper that NATO troops used.

Chemical weapons experts said it was possible that Novichok nerve agents had been used before on Kremlin targets in Britain but had escaped detection.

Exposure to Novichok nerve agents leads to muscle spasms, secretion of fluid into the lungs and organ failure, sometimes accompanied by foaming at the mouth. But if the victim has already died, experts said, police could easily mistake the cause of death for a heart attack.

“It’s entirely likely that we have seen someone expire from this and not realized it,” said Daniel Gerstein, a former senior official at the U.S. Department of Homeland Security who is now at the RAND Corp. “We realized in this case because they were found unresponsive on a park bench. Had it been a higher dose, maybe they would have died and we would have thought it was natural causes.”