CBS News says the former head of Russia‘s drug testing laboratory Grigory Rodchenkov has told whistleblower Vitaly Stepanov that at least four of Russia’s gold medal winners at the 2014 Winter Olympics were using steroids, according to an interview to be aired on Sunday.

Stepanov, who previously worked for Russia’s anti-doping agency and is now living in the United States, told the investigative programme “60 Minutes” that Grigory Rodchenkov, the former head of the drug laboratory, had evidence of the use of banned substances at the Games in Sochi, Russia.

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

The World Anti-Doping Agency (WADA) said the body was aware that Sunday’s programme would “include content regarding further doping allegations”.

“We will watch the programme with interest,” WADA spokesman Ben Nichols told Reuters.

“We will not comment further until we have viewed the programme in full.”

Russia is already banned from all track and field competitions, including August’s Rio Olympics, after an independent WADA commission last November revealed widespread state-sponsored doping.

Its athletes will be allowed to return to competition when Russia can prove that it has met several conditions regarding its anti-doping operation, WADA and the International Association of Athletics Federations have said.