British Prime Minister Theresa May says the chemical used to poison Russian ex-spy Sergei Skripal and his adult daughter in England was a Novichok nerve agent, tied to Russia not only through its name. Novichok—meaning "newcomer" in Russian—is a class of chemical agents developed in the Soviet Union in the 1970s and '80s "specifically to get around treaties banning chemical weapons," reports NBC News. Only revealed to the wider world by a former Russian scientist in 1991, the agents aren't fully understood, but they're believed to be among the most deadly in existence, per CNN. Though there are "no known previous uses," per Reuters, one specific agent, A-230, is described as five to eight times as toxic as VX, a weapon of mass destruction used in the February 2017 assassination of Kim Jong Nam, the half brother of North Korean leader Kim Jong Un.
In powder and liquid form, Novichok agents produce effects—respiratory failure and death by asphyxiation at worst—as little as 30 seconds after inhalation or absorption through the skin, reports the BBC. They're also believed to cause more permanent injury and be more resistant to antidotes than other nerve agents. In addition, the agents are easier to handle and harder to detect, as most involve two not-as-toxic components that only become deadly when mixed. This makeup apparently allowed the Soviet Union to develop such agents even after committing to chemical arms control, experts say. May says it's "highly likely" Russia is responsible for the March 4 attack on 66-year-old Skripal and his 33-year-old daughter, who remain in critical condition. But Russia is denying involvement and refusing to explain the use of Novichok until samples of the agent are provided, the AP reports. (Read more Sergei Skripal stories.)