The German government announced that it has identified Novichok as the poison used to incapacitate Russian opposition leader Alexei Navalny. It is the same nerve agent used to attack former spy Sergei Skripal and his daughter in Britain in 2018.

Navalny was moved on Aug. 21 from a hospital in Siberia to Berlin after Russian doctors said they had not found any evidence that he was poisoned. His ordeal began the day before when he fell ill on a flight back from Moscow.

A statement Wednesday by Steffen Seibert, spokesperson for German Chancellor Angela Merkel, announced that doctors had identified the specific toxin used as Novichok, a nerve agent. Seibert also said that he would be coordinating with other NATO allies for a “joint response.”

Russian officials continue to contest the results, saying that their tests came back negative and they have ruled out poisoning as a cause of Mr. Navalny’s illness.

President Valdimir Putin’s critics in Russia have raised an outcry over the incident, exacerbated by the repeated use of the agent, apparently to send a message.

Leonid Volkov, an ally of Navalny, said that “to poison Navalny with Novichok in 2020 would be exactly the same as leaving an autograph at a crime scene, like this one,” followed by an image of Putin’s name and signature.

Kremlin spokespersons have said that the information had not been relayed to them and that any accusations that Navalny had been poisoned were “empty noise.”

According to his doctors, Navaly’s condition is stable but he is still in a medically induced coma.