Opposition leader and Kremlin critic Alexei Navalny tricked a Russian agent who had tailed him prior to an attempt on his life in August into revealing that he was poisoned with a Novichok nerve agent planted in his underwear, Bellingcat reported Monday.

Posing as a fictitious aide to a senior Russian official, Navalny convinced reported Russian Federal Security Service (FSB) agent Konstantin Kudryavtsev, who Bellingcat previously exposed as a player in the plot to kill the Russian opposition leader, that he was taking part in a debriefing.

“What went wrong?” Navalny asked. “Why the Navalny operation in Tomsk was a complete failure?”

Navalny fell violently ill on August 20 while flying from Tomsk to Moscow. The aircraft made an emergency landing, and he was taken to a hospital in Omsk, where he was put in a coma. He was then taken to Germany for further treatment, and it was there that it was determined that he was poisoned with a nerve agent from the Novichok family, a collection of chemical weapons developed by the Soviet Union.