In the first days of 2021, seven months before the pandemic-delayed Tokyo Olympics, 23 of China’s best swimmers tested positive for the same banned drug at a domestic meet.

Chinese antidoping officials investigated and declared the case an unusual mass-contamination event that could be traced to the presence of a heart medication, trimetazidine, known as TMZ, in the kitchen of a hotel where the swimmers had stayed for a New Year’s event in late December 2020 and early January 2021.

The World Anti-Doping Agency, the global authority that oversees national drug-testing programs, looked into the episode but then accepted that theory and allowed China to keep the results secret.

All the while, the swimmers were allowed to continue racing, without suspensions or disqualifications. Some of the swimmers who provided positive samples went on to qualify for the Olympics, and to win medals — including three golds — for China. A few are favorites to win again at the Paris Olympics this year.

The incident would have remained in the shadows, a secret known only to a select few, had details of one of the most curious episodes in swimming not made their way out of the sealed files of the organizations trusted to keep sports fair.