Britain's Porton Down defense laboratory announced on Tuesday it was unable to verify the precise source of the nerve agent used against Russian ex-spy Sergei Skripal and his daughter.

British Foreign Minister Boris Johnson told DW in March that Porton Down had "categorical" evidence that Russia was the source of the agent.

Read more: Russia says spy poisoning could be distraction from Brexit problems

Russian spy poisoning: How it unfolded Ex-Russian spy poisoned 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.

Russian spy poisoning: How it unfolded Russia denies involvement 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."

Russian spy poisoning: How it unfolded Nerve agent suspected 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."

Russian spy poisoning: How it unfolded UK promises retaliation 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.

Russian spy poisoning: How it unfolded May gives Russia a deadline 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.

Russian spy poisoning: How it unfolded EU supports UK 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."

Russian spy poisoning: How it unfolded Russia calls UK bluff 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.

Russian spy poisoning: How it unfolded UK announces expulsions of diplomats 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.

Russian spy poisoning: How it unfolded France, Germany, UK, US blame Russia 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."

Russian spy poisoning: How it unfolded Russia expels British diplomats 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.

Russian spy poisoning: How it unfolded Putin dismisses claims as 'nonsense' "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."

Russian spy poisoning: How it unfolded UK says Novichok was used 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.

Russian spy poisoning: How it unfolded Mass Russian diplomat expulsions 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.

Russian spy poisoning: How it unfolded Poison on front door 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.

Russian spy poisoning: How it unfolded New Novichok victims 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. Author: Louisa Wright



Military-grade nerve agent

Gary Aitkenhead, the chief executive of the Defence Science and Technology Laboratory at the facility, told Sky News:

"We were able to identify it as Novichok, to identify that it was military-grade nerve agent."

"We have not identified the precise source, but we have provided the scientific info to government who have then used a number of other sources to piece together the conclusions you have come to."

The substance required "extremely sophisticated methods to create, something only in the capabilities of a state actor."

"We are continuing to work to help to provide additional information that might help us get closer to [the source] but we haven't yet been able to do that."

Russian diplomats return home amid Skripal poisoning row

Boris Johnson's claims: Johnson told DW in an exclusive interview in March that Russia was definitely behind the March 4 Salisbury attack. "They were absolutely categorical and I asked the guy myself, I said, 'Are you sure?' And he said there's no doubt. We have very little alternative but to take the action that we have taken."

Read more: Boris Johnson: Russia's position in Skripal case is 'increasingly bizarre'

Emergency meeting: The Organization for the Prohibition of Chemical Weapons said it will hold an emergency meeting in The Hague on Wednesday at the behest of Russia.

Russian President Vladimir Putin said in Ankara that, given the inability to trace the source of the agent, "the speed at which the anti-Russian campaign has been launched causes bewilderment." He said the nerve agent could be made in some 20 nations.

Yury Filatov, Russia's ambassador to Ireland, said that if the UK was unable to provide evidence of Russia's involvement then "there are ample grounds to assume that [they] are dealing with a grand scale provocation organized in London aimed to discredit Russia."

Russian retribution: Scores of Russian diplomats and spies were expelled from a series of British allies following the attack on the basis that Russia was behind the attack.

aw/jm (AP, AFP, Reuters)

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