The Swedish Foreign Ministry announced Monday it will summon the Russian ambassador in Stockholm on March 20. Stockholm wants explanations over Moscow’s “accusations that Sweden could be the source of the poison that was used” against ex-double agent Sergei Skripal in the UK. Swedish Foreign Minister Margot Wallstrom has called that claim “unacceptable and unfounded,” according to Reuters. Russia’s ambassador will be summoned “on account of the Russian accusations that Sweden could be a source of the nerve toxin used in the attempted murder in Salisbury,” a Foreign Ministry spokeswoman told Reuters on Monday. Britain has accused Russia of being behind the attack. Russia’s Foreign Ministry said the most likely source of the nerve agent was Britain itself, the Czech Republic, Slovakia, the US or Sweden.