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.

The FSB agent revealed that a recon team switched off the cameras at Navalny’s hotel, clearing the way for the operations team to plant the poison.

When he asked why Navalny survived the attack, Kudryavtsev said things would have ended differently if the plane had not made an emergency landing or if medical personnel had been slower to respond.

“If [the plane] had flown a little longer and they hadn’t landed it abruptly somehow and so on, maybe it all would have gone differently,” Kudryavtsev, who was part of the clean-up crew and not the hit squad, said. “That is, if it hadn’t been for the prompt work of the medics, the paramedics on the landing strip, and so on.”

CNN, which worked with Bellingcat on the latest investigation, reported that toxicology experts said Navalny would have probably died if the plane he was on had continued on to Moscow.

Russian President Vladimir Putin acknowledged last week that the FSB had been tailing Navalny but denied that the security service had been involved in an attempt on his life.

“The intelligence agencies of course need to keep an eye on him,” Putin said, “but that does not mean that he needs to be poisoned — who needs him? If they had really wanted to, they would have probably finished the job.”

Navalny tried to engage other Russian security service agents, but they hung up on him after realizing who he was.

This article was written by Ryan Pickrell and originally published on Business Insider.