What are Novichok agents and what do they do?

There’s also the choice of target — Skripal sold the identities of Russian agents to Britain’s MI6, and was released by Russia in a 2010 swap of ex-spies — as well as the weight of history: A U.K. inquiry said Russians were almost certainly behind the 2006 poisoning in London, with polonium, of Alexander Litvinenko, another ex-spy. Russia rejects the suggestion.