British Prime Minister Theresa May said that it was “highly likely” that the Russian government was behind the attempted assassination of a former Russian military intelligence officer turned spy for Britain’s MI6 on March 4th.

“It is now clear that Mr. Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia,” May said in the House of Commons. “The government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal.”

She went on to say that, either the incident was a “direct act of the Russian state against our country,” or evidence that Russia failed to maintain control over its arsenal of banned nerve agents. As a result, the Prime Minister explained, the Russian ambassador has been summoned, stating that she and the UK government are demanding an explanation by the end of the day on Tuesday.

“Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom, and I will come back to this House and set out the full range of measures we will take in response,” May said.

“We shall not tolerate such a brazen act to murder innocent civilians on our soil.”

According to Prime Minister May, the nerve agent in question belongs to a family of poisons called “Novichok,” (Russian: новичок) which is roughly translated as “newcomer”, “newbie” or “new guy.” It was produced in large quantities by the Soviet Union in the 1970s and 80s, and had a fearsome reputation among U.S. researchers and analysts. According to the testimony of Vil Mirzayanov, one of the chemists that helped develop the toxin prior to the fall of the Soviet Union, Russia retained a stockpile of the agent large enough to kill “hundreds of thousands of people.”

The incident May referred to took place on March 4th in the British town of Salisbury, where Sergei Skripal, 66, and his daughter were found unconscious on a bench in the shopping district of the city center. The two were hospitalized and have remained in critical condition ever since, while law enforcement, emergency services, and eventually, hundreds of troops descended upon the scene to manage the cleanup effort that was labeled a “major incident” by UK authorities.

Skripal had formerly served as a Colonel in Russian military intelligence before selling the names of Russian operatives conducting clandestine operations in the UK to MI6 for a reported $100,000. Skripal pled guilty to treason and was sentenced to 13 years in prison by Russian courts in 2006, but was granted asylum to the UK following a prisoner transfer brokered by the United States that saw the return of captured Russian spies, and the release of a handful of spies that had done work for American or British intelligence services.