Three Chinese swimmers among 23 involved in a drug scandal ahead of the Tokyo Olympics had tested positive for banned substances in separate cases several years earlier, the New York Times reported on Friday.
The Times said the three athletes – including two 2021 Olympic gold medallists and a current world record holder – tested positive for clenbuterol in 2016 and 2017.
Chinese authorities argued the three athletes had ingested the substance inadvertently through contaminated meat, and no disciplinary action was taken.
The World Anti-Doping Agency (Wada), said in a statement released on Friday that the three athletes in question were found to have levels of clenbuterol which were between "six and 50 times lower" than the minimum reporting level currently used by the agency.
Swimming's international governing body was made aware of the cases and did not take action after apparently accepting the explanation, the Times reported.
Wada director general Olivier Niggli said in a statement the case highlighted the problem of clenbuterol contamination in meat.
"The issue of contamination is real and well-known by the anti-doping community," Niggli said.
"Over the years, there have been thousands of confirmed cases of contamination in its various forms, including more than 1 000 for meat contamination in Mexico, China, Guatemala, Colombia, Peru, Ecuador and other countries.
"The athletes in question were three such cases. They were elite level swimmers who were tested on a very frequent basis in a country where meat contamination with clenbuterol is widespread so it is hardly surprising that they could be among the hundreds of athletes who also tested positive for tiny amounts of the substance.
"In each of these cases, the source of the clenbuterol was confirmed to be food contamination."
It was not made immediately clear why the three cases were not made public at the time.
The latest revelations come after the sporting world was rocked by reports in April which revealed that 23 Chinese swimmers had tested positive for a prescription heart drug, trimetazidine (TMZ), ahead of the pandemic-delayed 2021 Tokyo Olympics.
Chinese authorities said the athletes had ingested the substance unwittingly through contaminated food.
The United States Anti-Doping Agency (USADA) said Wada and China's anti-doping body had "swept those positives under the carpet", describing the case as a "potential cover-up."
Wada responded furiously to those accusations, insisting the case had been handled according to existing protocols and denying claims of a cover-up.
The agency said it had "concluded that it was not in a position to disprove" that contamination was the source of the TMZ and that given the circumstances, "the athletes would be held to have no fault or negligence."