MOSCOW (AP) — Russian opposition leader Alexei Navalny on Monday released a recording of a phone call he said he made to an alleged state security operative who revealed details of how the politician was supposedly poisoned.

Navalny fell sick on Aug. 20 during a domestic flight in Russia and was flown to Berlin while still in a coma for treatment two days later. Labs in Germany, France and Sweden, and tests by the Organization for the Prohibition of Chemical Weapons, established that he was exposed to a Soviet-era Novichok nerve agent.

Russian authorities have vehemently denied any involvement in the poisoning.

Last week, the investigative group Bellingcat released a report alleging that operatives from Russia’s FSB domestic security agency with “specialized training in chemical weapons, chemistry and medicine,” were “in the vicinity” of Navalny in the timeframe "during which he was poisoned.”

The investigation, conducted by Bellingcat and Russian news outlet The Insider in cooperation with CNN and German news outlet Der Spiegel, identified the supposed FSB operatives after analyzing telephone metadata and flight information.

Navalny, who is convalescing in Germany, said the report proved beyond doubt that FSB operatives tried to kill him on the orders of Russian President Vladimir Putin. On Monday, he posted a video on his YouTube channel Monday titled “I called my killer. He confessed."

The video showed him speaking on the phone with one of the alleged operatives. Bellingcat and other media outlets identified the man Navalny said he spoke with as Konstantin Kudryavtsev, a trained chemical-weapons specialist. The investigation alleged that Kudryavtsev traveled to Omsk — the Siberian city where the plane carrying Navalny when he became ill made an emergency landing and where the comatose politician first was hospitalized — several days after Navalny was airlifted to Berlin.

Navalny said he phoned the alleged FSB operative hours before the Bellingcat report was released, Navalny introduced himself as an aide to Russian Security Council Secretary Nikolai Patrushev and said he urgently needed to debrief the man on what had happened in another Siberian city, Tomsk, where the politician had been before he was supposedly poisoned.

The man on the other end of the call indicated that he was involved in the “processing” of Navalny's clothes so “there wouldn’t be any traces.” The clothes Navalny was wearing when he was hospitalized in a coma have not been returned to him.

While posing as a Security Council aide, the politician asked “what went wrong" and why Navalny survived the poisoning. The man on the other end replied “it would have all gone differently” if the plane hadn't made the emergency landing and “if (it had) not (been) for the prompt work of the ambulance medics on the runway."

The man also reported that among Navalny's clothing, his underwear bore the highest concentration of toxic residue.

The Associated Press was not able to independently verify the identity of the man with whom Navalny spoke in the video. Russian authorities have not yet commented on the recording, which received over 1.4 million views on YouTube within hours of being posted.

Earlier this month, Russian officials brushed off the investigation by Bellingcat and other media outlets.

Putin charged last week that the investigation relied on data provided by U.S. spy agencies. Its authors have denied any link to U.S. or any other Western intelligence services.

“It’s not some kind of investigation, it’s just the legalization of materials provided by U.S. special services,” the Russian leader alleged during his annual press conference. He said that means Navalny “relies on the support of U.S. special services.”

“It’s curious, and in that case, special services indeed need to keep an eye on him,” Putin said. “But that doesn’t mean that there is a need to poison him. Who would need that?”