British officials now believe the nerve-agent attack on a former spy in England had approval from the top levels of Russian government. Sources told CNN the nature of the attack—likely to have been carried out by applying the nerve agent to Sergei Skripal’s front door—shows that it was done by someone with key training in the lethal Novichok substance developed by the Soviet Union. The apparent sophistication of the attack has led British authorities to believe it could not have been carried out by a rogue group who simply got hold of the nerve agent in question, and must have had the approval at the top level of the Kremlin. The source added that the attack was likely intended to send a warning to other Russian dissidents who now live in the West. Russia has repeatedly denied any involvement in the attempted murder of Skripal and his daughter Yulia, which sparked mass expulsions of Russian diplomats and spies from countries across the Western world.