Three labs have independently confirmed that Russian opposition leader Alexei Navalny was poisoned with the Novichok nerve agent, Germany said Monday, renewing calls for Russia to explain the incident.

Germany said earlier this month it had “unquestionable proof” that Navalny, 44, was poisoned with Novichok when he fell ill after boarding a flight in Siberia on Aug. 20. Navalny was flown to Berlin in a coma after two days of treatment in the Siberian city of Omsk, where doctors say they have seen no traces of poison.

The German government said in a statement that it sought independent analysis of Navalny’s samples from the Organization for the Prohibition of Chemical Weapons (OPCW) as well as specialized laboratories in France and Sweden.

“Independent of the ongoing OPCW investigations, three laboratories have now independently demonstrated the presence of a nerve agent from the Novichok group as the cause of Mr. Navalny’s poisoning,” it said.

“We renew the call for Russia to explain itself,” the German government added.

Moscow has rejected suggestions that it was responsible for Navalny’s poisoning and dismissed calls to launch a criminal investigation into the incident. The Russian government has complained that Germany has been ignoring its request to see the evidence that Navalny was poisoned with Novichok.