The Justice Department has opened a criminal investigation into how antidoping authorities and sports officials allowed elite Chinese swimmers who had tested positive for a banned substance to escape punishment and win a slew of medals — including three golds — at the last Olympics, according to two people briefed on the matter and swimming’s international governing body.

       The decision to move forward with a criminal investigation is a dramatic escalation by the United States against the Chinese, world antidoping authorities and the Olympic movement, and will cast a shadow of criminality over the Summer Games, which are scheduled to begin later this month in Paris.

       Eleven of the swimmers who tested positive — and who have never been suspended for doping — are again members of the Chinese Olympic team. Several are favorites to again win medals.

       The disclosure about the investigation comes a little more than two months after The New York Times revealed that the World Anti-Doping Agency and Chinese antidoping authorities had declined to discipline 23 elite Chinese swimmers who tested positive for a banned drug in early 2021. The decisions not to suspend the athletes and to keep the positive tests secret paved the way for the swimmers to compete and win medals at the Games in Tokyo.

       The Times reported that the F.B.I. had learned in the past year about the positive tests and the decision to clear the athletes of wrongdoing and that federal investigators had taken steps in recent weeks to learn more about what occurred. But it was unclear if a full criminal investigation into the matter had begun.

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