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.

In October, a Bellingcat investigation revealed that Russia had been secretly running a chemical weapons program and that scientists involved in the program were in contact with state-operated assassins.

In his call with Kudryavtsev, Navalny asked how the poison was administered, asking specifically about clothing as the delivery medium.

“On which piece of cloth was your focus on? Which garment had the highest risk factor?” Navalny asked.

“The underpants,” Kudryavtsev replied, explaining that the poison was placed along the inside seams in the groin area. The operatives involved expected the poison to be absorbed into the body and not leave a trace.