On Monday, British Prime Minister Theresa May addressed Parliament, accusing the Russians of using a nerve agent to try to kill Sergei Skripal, a Russian military intelligence officer turned British double agent resettled in Salisbury after a spy swap, and his daughter, Yulia.

It is now clear that Mr. Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia. This is part of a group of nerve agents known as novichok. Based on the positive identification of this chemical agent by world-leading experts at the Defence Science and Technology Laboratory at Porton Down, our knowledge that Russia has previously produced this agent and would still be capable of doing so. — Prime Minister Theresa May

The person who understands the effects of novichoks best is Vil Mirzayanov, a scientist and later head of Foreign Technical Counterintelligence at the State Scientific Research Institute of Organic Chemistry and Technology (GosNIIOKhT) in Moscow in the 1970s and 1980s, which allegedly produced the shadowy class of binary nerve agents known as the “novichoks” (newcomers). And he has a message for Skripal and his daughter: my bad.

“I’d tell him [Skripal] that I’m very sorry that I participated in the development of these weapons,” Mirzayanov told The Daily Beast.

GosNIIOKhT scientists developed the agents under a program codename “Folio” beginning in the 1980s. Mirzayanov spoke out about the covert program as the Soviet Union fell, earning him a prison term at home before he escaped to exile in the United States.

During the Cold War, the idea that a novichok agent would be used in a covert assassination seemed alien to Mirzayanov and his fellow scientists. The weapons, developed in intense secrecy by Soviet scientists, were originally designed for use in bombs and shells on a battlefield rather than a cloak-and-dagger assassination in a suburb in southern England.

“I couldn’t imagine. No one could imagine. It’s outrageous. We were convinced at the time that we were developing these weapons and testing others for the protection of the country and for defense,” Mirzayanov said. “It was not our goal. None of the scientists supposed that it would be used with terrorist goals. It was a military thing. It was a weapon for mass killing.”

Despite the lethality of novichok agents—reportedly 10 times as powerful as the VX nerve agent used to assassinate Kim Jong Un’s brother in a Malaysian airport—Skripal and his daughter managed to survive the attack and are recuperating in the hospital. But Mirzayanov expects that the two have a long road to recovery.

“If you’re poisoned by a nerve agent, it’s forever,” Mirzayanov said, citing the case of his friend and fellow GosNIIOKhT employee, the late Andrei Zheleznyakov, who was accidentally poisoned by a novichok.

Zheleznyakov received a dose of A-232, a component used in Novichok No. 5, during a laboratory accident in the 1980s. Doctors gave Zheleznyakov atropine, a common nerve agent antidote, and he survived following a lengthy recuperation. But the once vibrant researcher was never the same, according to Mirzayanov: “He wasn’t capable of functioning normally afterwards.” Zheleznyakov reportedly suffered from chronic weakness, epilepsy, liver problems, and difficulty focusing, among other maladies, before his death five years later.

“ I’d tell him [Skripal] that I’m very sorry that I participated in the development of these weapons. ” — Vil Mirzayanov

In Skripal’s case, British investigators may have been able to identify the use of a novichok with help from a portable mass spectrometer, according to Mirzayanov. The scientific devices could detect the presence of a novichok if investigators had reliable information on the chemical composition of the nerve agents.

For the prime minister to be able to publicly accuse the Russians of using a nerve agent like a novichok, British authorities at least must have had access to novichok’s unique chemical signature—which it legally could have had despite the Chemical Weapons Convention, due to the clause of countries being able to hold samples for testing in these incidences.

Testing for novichoks, even based on a formula published by Mirzayanov in a memoir based on his work in the 1980s, is a potential sign that the British have potential access to newer variants of the nerve agent.

Gwyn Winfield—the editorial director of CBRNe World magazine, a trade publication for those dealing with chemical, biological, radiological, nuclear, or explosive threats—said it’s possible that the Russians continued to develop novichoks in the years after the publication of Mirzayanov’s book. But even then, the British were probably able to reliably gather minute traces with a novichok’s unique chemical signature.

“You’d have to detect the phosphate chain, and they [mass spectrometers] are probably looking at broad families detecting something hazardous in the environment,” Winfield added, referring to a flame phytometry detector that the British Army likely used for chemical identification.

Winfield said it’s also important to note that we don’t yet have any idea what particular novichok variant hit the Skripals and the police officer who responded to them.

W hen Winfield first heard about the incident, he and other journalists thought it was a fentanyl overdose. “When the Salisbury Hospital shut down, [it was done so for] fentanyl poisoning,” Winfield told The Daily Beast. “That sounded right, that we were looking at two individuals who had overdosed on something.”

Fentanyl wouldn’t be out of the question: Prior to its more recent use in the United States as an opioid, Winfield said the drug has been used for assassination attempts. “It was effective and inefficient, and there was plausible deniability,” he said, pointing out that victims could be thought to have overdosed.

When Prime Minister May came out and said the culprit was novichoks, Winfield said he was surprised, particularly given that Skripal reportedly became aggressive, waved his arms, and pointed to the sky while yelling in Russian, he said. “Those don’t fit into what we know about organophosphate exposures,” Winfield explained—which means that while novichok is being pointed to as the source nerve agent, it’s possible that it was swirled with another drug that produced hallucinogenic qualities that were more similar to a fentanyl poisoning.

“ If they wanted to kill me, it would’ve been easy. But God has saved me so far. ” — Vil Mirzayanov

But that’s impossible to confirm, Winfield said. “It might have been a cocktail of drugs, it might have been [Skripal’s] unique physiology,” he surmised.

The business of killing with chemicals is an ugly one and it weighed heavily on Mirzayanov’s conscience in the waning days of the Cold War. The former Soviet scientist once in charge of protecting GosNIIOKhT’s secrets from Western spies began a crusade to eradicate chemical weapons with a Moscow News article and an interview with The Baltimore Sun in 1992.

Mirzayanov told The Daily Beast that representatives from the U.S. intelligence community later urged him not to publish chemical formulas and compounds for the production of novichoks because of concerns about the dissemination of know-how for the powerful weapons.

“The U.S. government doesn’t like me because they can’t use me like a puppet,” he said. “I’m not a puppet. I refuse to cooperate the way they want. I told them, ‘Go to the court and challenge me.’ I believe Russian secrets are not American secrets.”

That we don’t know much about the subsequent development of novichoks after Mirzayanov had access to GosNIIOKhT’s secrets means it’s also hard for us to know how the Skripals got sick. “It could be a gel, vapor, or a liquid,” Winfield said. Nerve agents tend to use corrosive acids, which mean their precursors are usually stored separately, Winfield said, but it’s possible this novichok doesn’t require mixing of chemicals and could be directly applied to a person’s skin or on a surface with “just a pair of gloves,” he said.

Winfield said that at this point, there are more questions than answers about novichoks and how they were used in the Salisbury attack. That Skripal, his daughter, and the police officer lived is also puzzling, given that Mirzayanov said practically no human testing occurred, unlike other nerve agents like sarin and VX. The survival of the victims could be due to human incompetence or their unique physiology, or even the cooler temperatures, that saved them.

With Russian intelligence veterans like Alexander Litvinenko and Sergei Skripal in the crosshairs of assassins, some dissidents are now worried about their own safety. Mirzayanov, 83 years old and living openly in Princeton, New Jersey, is unmoved by the potential threat from the Russian government.

After he spoke out against GosNIIOKhT’s chemical weapons programs, Russian authorities arrested Mirzayanov and put him in the KGB’s notorious Lefortovo Prison in Moscow. He survived, walking out of a place where so many before him had entered but never left.

“If they wanted to kill me, it would’ve been easy,” he says now. “But God has saved me so far.”