On March 4, former Russian double agent Sergei Skripal and his 33-year-old daughter were found slumped on a bench outside a shopping center in the British town of Salisbury. Authorities said both were in a critical condition after being exposed to an "unknown substance." Skripal was a former general of Russian military intelligence who had been convicted in Russia for spying for the UK.

Russia denied any knowledge of the poisoning, which echoed the murder of former Russian spy Alexander Litvinenko in 2006. Litvinenko was poisoned with radioactive polonium-210. "We see that such a tragic situation happened," Russian President Vladimir Putin’s spokesman Dmitry Peskov told journalists on March 6. "But we don't have information about what could be the cause, what this person did."

On March 7, British police said they suspected a very rare nerve agent was behind the poisoning of Skripal. "This is being treated as a major incident involving attempted murder by administration of a nerve agent," Metropolitan Police assistant commissioner Mark Rowley said in a statement. "I can also confirm that we believe the two people originally who became unwell were targeted specifically."

British police said more than 21 people had sought medical treatment as a result of the nerve agent attack. On March 8, UK Home Secretary Amber Rudd told the House of Commons that enormous resources were being used to determine who was behind the attack. Rudd called the use of a chemical nerve agent on British soil a "brazen and reckless" act that would be answered with all possible force.

On March 12, British Prime Minister Theresa May told lawmakers it was "highly likely" Russia was behind the poisoning. May said the Russian government had either ordered the attack or lost control of the Russian-produced chemical nerve agent Novichok. She gave Moscow until midnight on Tuesday to explain its Novichok program to the Organization for the Prohibition of Chemical Weapons.

On March 13, vice president of the European Commission European Union, Valdis Dombrovskis, said the EU would stand in solidarity with Britain after London accused Russia of being behind the nerve agent attack. When asked if the EU might impose sanctions of Russia if it was agreed Moscow was responsible for the attack, Dombrovskis said: "Of course, the UK can count on EU solidarity in this regard."

Russia failed to respond to May’s midnight deadline for an explanation of its suspected involvement in the poisoning. On March 14, a spokesperson for the Russian Embassy in London said Moscow would not respond "until it receives samples of the chemical substance." May had said a "full range" of retaliatory measures would be considered if Moscow did not give a "credible response" by the deadline.

After Russia failed to give an explanation, May announced on March 14 that the UK would expel 23 Russian diplomats identified as "undeclared intelligence officers." May also said the UK would suspend all high-level bilateral contact with Russia. The biggest expulsions from London in 30 years would "fundamentally degrade Russian intelligence capability for years to come," May said.

On March 15, the leaders of France, Germany, the UK and US released a joint statement that demanded "complete disclosure" from Russia saying there is "no plausible alternative" to Moscow's involvement. The statement said the attack using "a military-grade nerve agent, of a type developed by Russia" constituted "an assault on UK sovereignty" that threatened "the security of us all."

In retaliation to the UK, Russia said it would also expel 23 British diplomats, giving them the same one-week deadline. Russia’s Foreign Ministry said it would also close the British Council in Russia, and might take further measures against Britain in the event of more "hostile steps" from London. Foreign Minister Sergey Lavrov, had said Moscow would "of course" respond with expulsions.

"It's complete drivel, rubbish, nonsense that somebody in Russia would allow themselves to do such a thing ahead of elections and the World Cup," Putin said on March 19. "It's quite obvious that if it were a military-grade nerve agent, people would have died on the spot." Putin said Moscow "destroyed all our chemical weapons under international oversight unlike some of our partners."

On March 20, UK scientists determined Skripal was poisoned using a little-known nerve agent from a group of chemical compounds known as Novichok. The family of compounds, which were developed in the 1970s and 80s, comprise numerous nerve agents. The Soviets once developed these weapons to circumvent the Chemical Weapons Convention. Novichok-5 and Novichok-7 are supposed to be the most dangerous.

A number of EU countries teamed together on March 26 and simultaneously announced they would be expelling Russian diplomats. Germany, France, Poland, Italy, Denmark, the Netherlands, Latvia and Ukraine all announced they would be expelling Russian envoys. The US followed suit with the expulsion of 60 Russian diplomats and announced the closure of Moscow's consulate in Seattle.

UK police found the highest concentration of the nerve agent on the front door of the Skripal's family home in Salisbury. They believe that is where Skripal and his daughter must have first come into contact with the poison. It was likely mixed in with a "gloopy substance" smeared on the door handle.

In early July, weeks after both Skripals were discharged from the Salisbury hospital, another two people were apparently poisoned with the same substance in the nearby town of Amesbury. A 45-year-old man and a 44-year-old woman were found unconscious and were transported to the same hospital in critical condition.