Russian opposition leader Alexei Navalny duped a Russian FSB state agent into revealing details of an attack on him with the nerve agent Novichok, the investigative group Bellingcat reports.
Mr Navalny reportedly impersonated a security official to call the agent.
The agent, Konstantin Kudryavtsev, told him the Novichok had been placed in a pair of Mr Navalny's underpants.
Mr Navalny, who is still recovering in Berlin, posted a recording of the long conversation on his YouTube channel, external.
He collapsed on board a Russian airliner in August in the attack, which nearly proved fatal.
As part of Mr Navalny's ruse to elicit more details of the assassination attempt, Bellingcat says the call, external to Mr Kudryavtsev was set up to indicate it was coming from a Federal Security Service (FSB) landline.
In the conversation, Mr Navalny posed as a senior official seeking details for a report on the FSB operation.
Mr Kudryavtsev told him the swift response of the airline pilot and the emergency medical team in Omsk, Siberia - where Mr Navalny was first treated - could have been the reason for the failure to kill him.