After the revelation in April that 23 elite Chinese swimmers had tested positive for a banned substance months before the last Summer Olympic Games, China and the global anti-doping authority vigorously defended their decisions to allow them to compete in the Games in 2021. The swimmers, they insisted, had not been doping.

But as they made those claims, China and the anti-doping authority were both aware that three of those 23 swimmers had tested positive several years earlier for a different performance-enhancing drug and had escaped being publicly identified and suspended in that case as well, according to a secret report reviewed by The New York Times.

In both instances, China claimed that the swimmers had unwittingly ingested the banned substances, an explanation viewed with considerable skepticism by some anti-doping experts. The two incidents add to long-standing suspicions among rival athletes about what they see as a pattern of Chinese doping and the unwillingness or inability of the global authority, the World Anti-Doping Agency, to deal with it.