LONDON — After asserting on Monday that it was “highly likely” that Russia was behind a poison attack against a former double agent in a quiet English town, Prime Minister Theresa May gave the Russian government a deadline to explain itself and where the rare and powerful “weapons-grade” nerve agent came from.

As expected, May’s deadline passed on Wednesday and Russia did not respond — or did not respond with the details or explanations that Britain sought.

Instead, Russian officials and state media assailed the British for whipping up “anti-Russia hysteria.” The Kremlin rejected the “unfounded accusations” and shrugged off British demands.

May vowed that without a “credible response” from Moscow, she would return to Parliament on Wednesday “to set out the full range of measures we will take.” Her office declined to give a time for May’s announcement of reprisals.

British politicians and commentators said May could employ a range of diplomatic and financial sanctions — from clamping down on Russian oligarchs’ property-buying binge in London to tossing out embassy staff.

May could also ask the European Union, or even NATO, to join in a response to what she described as a “reckless” and “indiscriminate” attack, which not only endangered the lives of its two principal victims, the former Russian spy Sergei Skripal, 66, and his daughter, Yulia, 33, but also potentially exposed scores of others to the nerve agent, including a police officer who remains hospitalized.

Skripal was jailed in Russia in 2006 for selling state secrets to British intelligence for 10 years, but he was released in 2010 as part of a high-profile spy swap. He and his daughter remain in critical condition at a Salisbury hospital.

A spokesman for 10 Downing St. said the British leader discussed the attack with President Donald Trump, who said Washington was “with the U.K. all the way” that Russia “must provide unambiguous answers as to how this nerve agent came to be used.”

In his last remarks, just hours after being fired by Trump via Twitter, Secretary of State Rex Tillerson warned, “much work remains to respond to the troubling behavior and actions on the part of the Russian government.”

Tillerson warned, “Russia must assess carefully as to how its actions are in the best interest of the Russian people and of the world more broadly. Continuing on their current trajectory is likely to lead to greater isolation on their part, a situation which is not in anyone’s interest.”

May also spoke with German Chancellor Angela Merkel. “They agreed that the international community should coordinate closely as the investigation developed and in the wake of Russia’s response,” her spokesman said.

Russia essentially blew off May’s midnight deadline for an explanation of how deadly Novichok nerve gas appeared on the streets of the quiet medieval town of Salisbury, famous for its nearby ruins of Stonehenge.

Various officials and commentators made it clear that Moscow would call her bluff.

After Russian Foreign Minister Sergei Lavrov told the U.K. on Tuesday that Moscow had no intention of responding to May’s ultimatum, the ministry’s spokeswoman, Maria Zakharova, dialed up the heat on an evening talk show on one of the leading state-run channels.

“No one can come before their national parliament and say: I give Russia 24 hours,” Zakharova said. “What kind of conversation is that in principle?” She then appeared to chastise London for not behaving like a nuclear power, and took a shot at Boris Johnson.

“When a country’s Foreign Ministry is led by people who have absolutely nothing to do with foreign policy, who have built their career around populism, they have no idea either about the organization for the prohibition of nuclear weapons or the relevant [chemical weapons convention].”

“To them, it is normal to go out and start intimidating,” Zakharova said. “Don’t. There is no need.”

In a conference call with journalists in Moscow Wednesday, Kremlin spokesman Dmitry Peskov said that theories surrounding Skripal’s poisoning are not the Kremlin’s problem. He reiterated that Moscow’s official position, that it was not involved and demands proof, has been delivered through diplomatic channels.

Peskov said that Moscow does not accept London’s accusations and hopes the West will come to their senses and engage Russia in a joint investigation into the poisoning of Skripal.

Regarding possible British actions against Russia today, Peskov said that “any unlawful actions against any Russian media outlets in the U.K. will, of course, lead to reciprocal measures backed the principle of reciprocity.” So far, no one in Russia has specified which outlets a response would apply to, thought they have suggested that every British outlet could be targeted.

So far, Lavrov is the most senior Russian official to comment on May’s ultimatum. President Vladimir Putin, according to his press service, was traveling to Russia’s southern Dagestan Republic.

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