The United Kingdom has issued a deadline to Moscow to explain why a Russian-made nerve agent was used in an attack on a former spy in Salisbury earlier this month. On Monday, British Prime Minister Theresa May said it was “highly likely” that Russia was behind the March 4 attack in Salisbury.

Russia now has until midnight (local time), BBC reported. The UK government has summoned the Russian ambassador to explain what happened. May said she will have to conclude there has been an “unlawful use of force” by Moscow if they do not respond. May made the remarks in Parliament on Monday, after which Russia responded angrily, calling it a “provocation” and a “circus show in the British parliament”, The Guardian reported.

The British prime minister explained that Russia had previously produced such a nerve agent and that it was still capable of doing do. Given this, she said, the UK was left with two possible explanations: “Either this was a direct act by the Russian state against our country, or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others.”

The attack

On March 4, Sergei Skripal, a former Russian double agent, and his daughter Yulia were found unconscious on a bench outside the Maltings shopping centre in Salisbury. Both of them, and a police officer who first responded to them, are said to be in serious condition in hospital. Police said the substance that rendered them critically ill was a nerve agent.

UK health officials on Sunday asked hundreds of people who visited a restaurant and a pub where the attack took place to wash their clothes and wipe personal items like handbags and phones, in case the substance came in contact with them.

Toxic nerve agents enter the body through nose or mouth and attack the nervous system, subsequently stopping it from working.