In a joint statement on Thursday, the United Kingdom, the United States, France and Germany held Russia responsible for the nerve agent attack on a former double agent in Salisbury, BBC reported. The nations said Russia’s involvement was the “only plausible explanation” for the “first offensive use of a nerve agent in Europe since World War II”.

The North Atlantic Treaty Organisation and the European Union also expressed their solidarity with the United Kingdom.

The four nations called on Russia to submit all information about the attack. The joint statement said: “It is an assault on United Kingdom sovereignty and any such use by a state party is a clear violation of the Chemical Weapons Convention and a breach of international law. It threatens the security of us all.”

Earlier on Thursday, Russian Foreign Ministry said Moscow would respond to the United Kingdom’s expulsion of 23 diplomats with expulsions of its own, Reuters reported. The United Kingdom’s expulsion announcement followed Moscow’s rejection of British Prime Minister Theresa May’s ultimatum to respond to her allegations that Russia was involved in the attack. It was the biggest expulsion in three decades.

Russia’s Foreign Minister Sergei Lavrov had rejected claims about Russia’s role in the attack. Lavrov had said Russia’s requests to see the samples of the nerve agent had been turned down which, he claimed, violated the chemical weapons convention that outlaws the production, stockpiling, and use of chemical weapons. He said under the convention, Moscow has 10 days to reply to an official accusation.

The attack

Skripal and his daughter Yulia were found unconscious on a bench outside the Maltings shopping centre in Salisbury on March 4. 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.