On Wednesday, Germany said that Russian opposition leader Alexei Navalny was poisoned with a substance from the deadly Novichok family. Novichok is a military-grade poison that was developed by the Soviet government toward the end of the Cold War.
The news sent shockwaves around the West.
Here's an overview of how the world's leading officials have so far reacted to the news:
German Chancellor Angela Merkel demanded answers from Russia over the case after Berlin said it had proof he was poisoned with a Novichok nerve agent.
"This has given rise to some very serious questions that only Russia can and must answer," Merkel said.
The German government said Navalny, who fell ill in Siberia in August and was later transferred to Berlin for treatment, had been poisoned with a nerve agent from the Novichok family.
EU chief Ursula von der Leyen on Wednesday denounced what she called the "despicable and cowardly" poisoning of Russian opposition leader Alexei Navalny.
Those responsible should be brought to justice, she said
"I was informed by Chancellor (Angela) Merkel that Russian opposition leader Navalny was attacked with a nerve agent, in his own country," von der Leyen tweeted.
The White House said Wednesday it is "deeply troubled" by Navalny's poisoning.
"Alexei Navalny's poisoning is completely reprehensible," National Security Council spokesman John Ullyot said on Twitter, noting that the poison was Novichok, a nerve agent used by Russia in the past.
"We will work with allies and the international community to hold those in Russia accountable, wherever the evidence leads, and restrict funds for their malign activities," he said.
"The Russian people have a right to express their views peacefully without fear of retribution of any kind, and certainly not with chemical agents."
British Prime Minister Boris Johnson said Wednesday it was "outrageous" that Navalny had been poisoned with a nerve agent and urged Moscow to "explain" what had happened.
"We have seen first-hand the deadly consequences of Novichok in the UK," Johnson said on Twitter.
The same type of nerve agent was used in Britain against ex-double agent Sergei Skripal, in an attack London blamed on Russia but Moscow denied any involvement in.
"The Russian government must now explain what happened to Mr Navalny, we will work with international partners to ensure justice is done," Johnson said.
Britain's Foreign Minister Dominic Raab said in a separate statement that Russia "must tell the truth" about what had occurred.
"The Russian government has a clear case to answer. It must tell the truth about what happened to Mr Navalny," Raab added.
France on Wednesday said that the use of the nerve agent Novichok in the poisoning of Navalny was shocking and irresponsible, adding that Russia had grave questions to answer in the case.
"I want to condemn in the strongest terms the shocking and irresponsible use of such an agent," Foreign Minister Jean-Yves Le Drian said in a statement after Germany, where Navalny is being treated after falling ill in Siberia, said he had been poisoned with Novichok.
Calling on Russia to explain what had happened, he added: "Given Mr Navalny's political status in Russia, the attack against him raises serious questions. It is the responsibility of the Russian authorities to respond to them."
AFP contributed reporting.