CBS News reported that whistleblower Vitaly Stepanov has tapes of the former head of Russia’s drug testing laboratory telling him that at least four Russian gold medallists at the 2014 Winter Olympics in Sochi were using steroids.

Stepanov, who previously worked for Russia’s anti-doping agency, told investigative programme “60 Minutes” that Grigory Rodchenkov informed him in over 15 hours of taped conversations he had evidence of a Sochi Games testing cover-up, which included the use of Russian intelligence agents.

Reuters was unable to independently confirm the disclosures in the “60 Minutes” report.

Stepanov said Rodchenkov told him “that FSB agents worked as doping controls officers during the Sochi Games, that FSB tried to control every single step of the anti-doping process in Sochi.”

FSB refers to Russia’s Federal Security Service.

CBS said it had listened to all 15 hours of tape and that at one point, Rodchenkov told Stepanov he was in possession of what he called “the Sochi list” of four Russian gold medal winners who were doping.

WADA spokesman Ben Nichols said the allegations about use of Russian intelligence in a doping cover-up and that four gold Sochi gold medallists were doping were “disturbing”.