LONDON—For nearly three decades, since a Soviet whistle-blower 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, collapsed onto a bench in a catatonic state on March 4, and remain hospitalized, in critical condition.

In a rare joint statement Thursday, British Prime Minister Theresa May, U.S. President Donald Trump, French President Emmanuel Macron and German Chancellor Angela Merkel and said “there is no plausible alternative explanation” to Russian responsibility for the attack.

“This use of a military-grade nerve agent, of a type developed by Russia, constitutes the first offensive use of a nerve agent in Europe since the Second World War,” the leaders said, calling it “an assault on U.K. sovereignty” and “a breach of international law.”

Earlier this week Britain’s Home Ministry indicated that it viewed state-sponsored violence by Moscow as a larger problem, announcing that it would scrutinize a series of suspicious deaths of Russians on British soil. Home Minister Amber Rudd said the police and MI5, Britain’s domestic security agency, would review 14 cases catalogued last year in an investigation by BuzzFeed. The British police also announced an investigation into the death on Monday of Nikolai Glushkov, a close associate of one of Putin’s most prominent foes.

In interviews, 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, either by inhalation or through the skin, 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, the police could easily mistake the cause of death for a simple 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 United States Department of Homeland Security who is now at the RAND Corporation. “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.”

The Russian foreign minister, Sergey Lavrov, said on Tuesday that his country had nothing to do with Skripal’s poisoning, adding that Moscow “had received an incoherent response” when it asked London for details, which he said amounted to a “rejection of our legitimate demands.”

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The dispute between the two countries has sharply worsened tensions between Russia and the West, already strained by Moscow’s role in the Syrian conflict and its annexation of Crimea from Ukraine.

Though American 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.

A chemist who worked in the laboratory developing Novichok accidentally inhaled fumes while filling a syringe, and collapsed. Though he was injected with an antidote and eventually awoke, he suffered from depression and epilepsy and died five years later, leaving Vil Mirzayanov, a scientist who helped develop the agent, deeply disillusioned.

“Antidotes exist, but what does antidote mean?” Mirzayanov, who had leaked the project to the press and later immigrated to the United States, told Sky News on Tuesday. “You’re saving a person who has been exposed to this gas — but temporarily, not to die this time. But he will be an invalid for the rest of his life.”

Andrew Weber, a former assistant secretary of defence for nuclear, chemical and biological defence programs, recalls picking his way through a secret, abandoned Soviet research facility in Nukus, Uzbekistan, which the United States was asked to helped destroy in the early 2000s.

Entering a basement room, Weber saw a disturbing sight: “dozens and dozens of restraining devices” used to immobilize dogs while their skin was exposed to Novichok agents in the form of a powder or paste. He said that he believed each test involved 50 to 100 dogs, and that at least 1,000 dogs had been killed at the facility.

The Pentagon, Weber said, “devoted a lot of resources to improving our protection, detection and countermeasures against it.” But it did not anticipate its use in an assassination, he said, in part because it was so easily traced to Russia.

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“It’s obviously tightly controlled by the Russian government,” he said. “It’s implausible to me — possible, but not probable — that this chemical weapon would have been diverted from a Russian facility. It would be well guarded.”

Dan Kaszeta, a chemical weapons specialist who once served as an adviser to the Secret Service, said the agents were “so shrouded in mystery that I don’t know how many chemical compounds are in the Novichok family.”

In Salisbury, where Skripal and his daughter were stricken, residents described mounting anxiety on Tuesday as they learned more about the nerve agent. The authorities reassured residents that there was no significant health risk. But they ratcheted up their precautions as the days went on, finally advising people who had been near the victims to wash or wipe everything they were wearing or carrying at the time.

Adam Langley, 44, a construction worker, said his 13-year-old daughter had been peppering him with questions, among them: “Is it true that it can stay in your body for years, and make you sick later?”

“She sends me texts throughout the day, asking if I’m sure we’re not going to die,” Langley said. He recalls swearing aloud when he researched Novichok on the internet.

Lisa Patterson, a local estate agent, had a similar reaction. “We’re not talking about rat poison,” she said, with a nervous laugh. “This stuff could kill a herd of elephants.”

Kaszeta, who now heads a British-based security firm, said the nerve agent could have been transported in a glass jar and spread on Skripal’s steering wheel, or on items he handled at a restaurant. The agent then could be transferred to anything Skripal touched for the next two hours, he said.

While the chemical would take effect “almost instantaneously” if inhaled, Kaszeta said, it would work much more slowly, perhaps over a matter of hours, if absorbed through the skin. The agent is activated when it comes in contact with water and would be absorbed through the pores, slowed down by subcutaneous fat, Kaszeta said.

At first the effect would be felt locally, around the point of exposure.

Once the chemical entered the bloodstream, it would cause the victim’s muscles to go into spasms, pupils to shrink to pinpoints, and breathing to become very laboured, said Alastair Hay, an emeritus professor of toxicology at the University of Leeds. At this point, the victim’s life could be saved only by the administration of atropine, which counteracts the agent and allows the body to metabolize it.

The attack on Skripal and his daughter occurred a short drive from Porton Down, Britain’s premier chemical weapons laboratory, which went to work isolating the agent from blood samples, breaking it into fragments and examining it through a mass spectrometer. Researchers would have initially looked for more common chemical agents, like sarin, and then proceeded through a long series of more obscure ones until they found a match, Hay said.

“When they get an unknown chemical, they will compare it with the information that’s in the library and, bingo, you’ve got all your strawberries lined up,” he said. Given how lethal the agent is, Kaszeta said, it seems probable that the two victims survived by happenstance.

“There are a few ways this could play out, and one is that something got screwed up in the delivery,” he said. “The other is that he washed his hands and got most of it off. The third is that this dosage was sublethal, just to send a message. It could have been the horse head in the bed.”

With files from the Associated Press