May said the Russian government had until midnight Tuesday to explain what happened and if they couldn't give a satisfactory explanation she would deem it the "unlawful use of force" by the Russian government against her country.
The Russian Foreign Ministry replied, saying that it would "not respond to London's ultimatum until the Russian side is provided with samples of a chemical substance referred to by the British investigation."
This confrontation highlights a fascinating contrast between the heads of the US and UK governments.
Whatever happens, the British Prime Minister will likely take stronger actions against the Russians then President Trump, who continues to treat Vladimir Putin as a trusted interlocutor rather than the dyed-in-the-wool KGB apparatchik he always has been. As recently as November, Trump said of Putin, in regard to the 2016 US election, "He said he didn't meddle...Every time he sees me, he says, 'I didn't do that,' And I believe, I really believe, that when he tells me that, he means it."
If that isn't dereliction of duty, I'm not sure what is.
By contrast, May has an opportunity to show strength as a leader against Putin.
May has been here before. She was the British home secretary during an inquiry into the death of another former Russian spy, Alexander Litvinenko, who died in 2006 from a lethal dose of the radioactive element polonium. After that attack, the British government, then controlled by the Labour Party, expelled Russian diplomats, but otherwise there were no larger repercussions.
This time feels different. Since the murder of Litvinenko, the Russian government has increasingly acted like a rogue state on the international stage, invading neighboring Ukraine in 2014 and two years later interfering in the US presidential campaign to help the candidacy of Trump, according to the collective judgment of multiple US intelligence agencies.
The British can choose from a menu of options that range from the aggressive to the largely symbolic.
The most aggressive approach would be to characterize the nerve agent attack as an act of war and to try and trigger NATO's Article 5 right to collective self-defense, which is what the United States did after the 9/11 attacks. This enabled NATO states to join the US-led coalition fighting the Taliban and al Qaeda in Afghanistan. Trying to trigger Article 5 for the nerve agent operation would be overkill.
This would allow the British to respond with, say, a large-scale cyberattack against Russian government websites, as well as against Russian government proxy websites that push disinformation.
The British certainly have the capability of carrying out such an attack, but there are, of course, risks in amping up tensions between the UK and Russia, which are both nuclear-armed states. But this is an option being seriously considered by the Prime Minister, according to The Times.
If the British government determines that the Russian state was responsible for the nerve agent attack, this option has considerable merit as it would signal to Putin that the era of virtual impunity he has enjoyed when it comes to attacking the West is now over.
The British government could also seek European Union help to ramp up sanctions against the Russians, but given Brexit, the British don't have a strong hand dealing with the EU and a number of European states have no intention of angering the Russians.
One likely scenario is that Ofcom, the UK's communications regulator, revokes the license for RT, Russian state television, so that it can no longer operate in the UK.
Then there is always low-hanging fruit, such as the expulsion of Russian diplomats from the UK. This almost certainly will happen, but the British government clearly wants to do more.
One approach could be to sanction by name Russian officials and businessmen who are close to Putin, preventing them from doing business or traveling to the United Kingdom. This would certainly get the Kremlin's attention, as the UK is almost a second home for much of the Russian elite who -- irony alert -- appreciate its rule of law culture.