The United States announced on Monday that President Trump has ordered the expulsion of 60 Russian diplomats, joining a bevy of other European nations taking similar actions following the use of a Soviet-era nerve agent on a former Russian spy on British soil.

Although UK officials have permitted international access to the substance recovered from the site of the attack in Salisbury, England, their own tests have confirmed that the substance used to poison Sergei Skripal, a former Russian intelligence officer turned British informant, and his 33-year-old daughter on March 4th, was indeed Novichok — a nerve agent developed and produced by the Soviet Union in the 1980s.

Novichok was created specifically to circumvent existing treaties forbidding the use of chemical weapons and nerve agents. Designed to use no substances included on the banned chemical weapons lists, Novichok is transported as a binary agent that is incredibly difficult to detect, but once combined, becomes one of the most deadly nerve agents ever concocted.

Despite Moscow’s varied, and often contradictory, claims as to how a specifically Soviet weapon was used in an attempted assassination against a man that betrayed the Russian government, British Prime Minister Theresa May has been steadfast in her condemnation of Russia’s involvement in the matter. Soon after the incident, which saw more than 30 other British citizens hospitalized after being exposed to the nerve agent, Theresa May issued an ultimatum to the Russian government: explain yourselves or there will be consequences.