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A documentary from German broadcaster ARD, plus reports from the New York Times and News Corp, have revealed 23 Chinese swimmers tested positive to the same banned substance seven months before the Tokyo Olympics.
While World Anti-Doping Agency and World Aquatics were informed at the time, the news wasn't publicly released and the athletes weren't punished.
So what actually happened, and why weren't they found guilty of doping?
Claims of a cover-up
The 23 athletes tested positive to a banned substance known as trimetazidine (TMZ).
It's a drug used to treat heart disease but is considered performance enhancing as it can help with physical endurance.
Chinese swimmer Sun Yang served a three-month doping suspension in 2014 for taking TMZ, while teenage Russian figure skater Kamila Valieva received a four-year ban after she tested positive to the same substance at the 2022 Beijing Winter Olympics.
According to an investigation by the China Anti-Doping Agency, CHINADA, as detailed in ARD's documentary, traces of the doping agent were found in the hotel kitchen where the team was staying.
The traces were found in the extractor fan, spice containers and the drain.
"That a doping pill ends up in whole or in part in a pot of soup, from there onto plates and then in the stomachs of athletes does indeed sound very contrived," filmmaker Hajo Seppelt said.
"The fact that the World Anti-Doping Agency does not want to challenge such a declaration casts a shadow on the credibility of the international anti-doping fight three months before the Olympic Games in Paris," he added.
Speaking to ABC RN Breakfast, USADA chief executive Travis Tygart accused WADA and CHINADA of a cover-up.
"It's terrible for the anti-doping system, and all WADA had to do when they were handed that report in June (2021) is do what the rules require," he said.
"If they wanted to find it was contamination, then OK, find it's contamination. But you still have to announce that, you still have to disqualify the results. And don't sweep it under the carpet."
Australian Swimmers Association president Mack Horton, who refused to share a podium with Sun Yang at the 2019 World Championships, told Nine Newspapers: "This news is infuriating for the entire sporting community."
"The news compromises the integrity of sport in general, not just swimming."
Due to COVID-19 restrictions, WADA couldn't investigate the case on the ground at the time, but in a statement said: "The Agency still stands firmly by the results of its scientific investigation and legal decision concerning the case.
"Based on all available scientific evidence and intelligence, which was gathered, assessed and tested by experts in the pharmacology of trimetazidine (TMZ); and, by anti-doping experts, the Agency had no basis under the World Anti-Doping Code to challenge the China Anti-Doping Agency's (CHINADA's) findings of environmental contamination – a position that was also accepted by World Aquatics."
WADA held a media conference early on Tuesday morning (AEST) to address the fallout from the revelations, defending its actions.
"At every stage WADA followed all due processes and investigated every lead and line of inquiry in this matter," WADA president Witold Banka said during a nearly two-hour Zoom call.
"If we had to do it over again we would do exactly the same thing.
"We carefully reviewed the decision of the Chinese anti-doping organisation from every perspective, we interrogated every piece of evidence and gathered further information.
"In this particular case, we followed the process and we don't see room for improvement when it comes to this particular process what we decided."
Contamination cases
There have previously been high-profile cases attributed to contaminated food or supplements.
In 2019, Australian swimmer Shayna Jack was suspended for four years after testing positive to the banned substance Ligandrol.
The Court of Arbitration for Sport eventually halved the ban to two years after finding she didn't intentionally ingest the drug.
Jack suggested she could have taken contaminated supplements.
Karen Brown is a former elite netballer, pharmacist and co-founder of Batch Tested, which helps athletes and sport teams test supplements for contamination.
"One in five supplements that you find in the supermarket or the pharmacy contain a prohibited substance that would result in a positive drug test for an athlete," she told ABC Sport.
"Our mission is that no Australian athlete returns a positive drug test.
"The last cycle, which was 2022-2023, was the first 12-month cycle where there were zero positive drug tests.
"Previous to that it was up to one a month, and in some cycles it was 16, 17 positive drug tests for these contaminated supplements.
"I think the supplement companies have gotten better [at testing], and athletes are now getting more educated on not taking that risk and making sure that they do identify that it's tested or not tested."
Many athletes have also tested positive to banned substances due to alleged food contamination.
"It's more common from a steroid perspective," Ms Brown said.
"And that comes from meat where the animals have legally been able to be given steroids, but that has then crossed into the food for the athlete.
"I know athletes have been advised against street food in particular, and making sure that they eat from a reliable source."
How could a banned substance get into the kitchen?
Ms Brown has raised questions about how TMZ ended up in the Chinese hotel kitchen.
"Even if a particular worker was using the medication for their own health reasons, it'd be unlikely that it would be found contaminated across so many surfaces," she said.
WADA's senior director of science and medicine said it was possible that somebody using TMZ and working in the kitchen "could have very well contaminated the areas".
"The fact that the spice containers were found with traces of trimetazidine in the kitchen as well, is not incompatible with somebody [using] trimetazidine on site [who could] have contaminated those elements," Professor Olivier Rabin said.
Despite Rabin's comments, WADA general counsel Ross Wenzel stressed that exactly how TMZ ended up in the kitchen is not known, with Chinese anti-doping authorities' interviews not coming up with "anything concrete".
"And they didn't hypothesise in their report," Wenzel said.
Why weren't they banned?
There have been some questions raised about the fact the Chinese swimmers weren't punished.
In many cases, athletes will receive a provisional ban, even while their case is being investigated.
Australian runner Peter Bol was provisionally suspended last year after testing positive to EPO.
The ban was lifted after his B sample returned an atypical finding, meaning it wasn't positive or negative.
But World Aquatics Doping Control Rules state athletes can avoid suspension if it's found they bear no significant fault or negligence, and that the detected prohibited substance came from a contaminated product, including food.
Wenzel said the agency had no evidence of "anything untoward" going on, and Banka they had "no credible way to disprove the contamination theory".
"In short, if you had taken such an appeal and challenged the contamination explanation, we would certainly have lost," Banka said.