The F.B.I. and Justice Department have 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 mean the shadow of an F.B.I. investigation will hang 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 win medals in Paris.

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.

Last month, when the executive director of the international governing body for swimming, World Aquatics, was in the United States for the U.S. Olympic trials in Indianapolis, federal investigators approached him to discuss how the positive tests were handled, according to two people briefed on the matter. Those people declined to be identified while discussing an ongoing investigation.

It is unclear what the swimming body’s executive director, Brent Nowicki, told the authorities in their interaction. Mr. Nowicki moved into his post in June 2021, a few days before Chinese authorities informed the World Anti-Doping Agency and World Aquatics that it had decided not treat the positive tests as doping violations.

As part of his dealings with the investigators, Mr. Nowicki was given a grand jury subpoena, according to a statement from World Aquatics.

“World Aquatics can confirm that its executive director, Brent Nowicki, was served with a witness subpoena by the United States government,” World Aquatics said. “He is working to schedule a meeting with the government, which, in all likelihood will obviate the need for testimony before a grand jury.”

The Associated Press first reported the statement from World Aquatics on Thursday.

Other evidence has emerged that officials at the World Anti-Doping Agency, known as WADA, are aware of ongoing law enforcement action directed at them by the American authorities. Late last month, WADA canceled a meeting it was scheduled to have in the United States later this year with other antidoping authorities and sports officials.

On a call to announce the cancellation of the meeting, a WADA official said that among the reasons it was being canceled was that the organization’s leaders did not want to travel to the United States because of an ongoing federal law enforcement investigation, according to Travis Tygart, the head of the United States Anti-Doping Agency. Mr. Tygart — who has been one of WADA’s most outspoken critics — was not on the call but one of his deputies was, he said.

The federal investigation is being conducted, partly, by prosecutors and F.B.I. agents in Boston, according to two people briefed on the matter. The authorities have interviewed at least two witnesses, according to the two people.

In a statement, WADA said it was “disappointed to learn that the U.S. Department of Justice is now investigating” the positive tests, adding that it “has not received any contact or request from U.S. law enforcement.”

Similar to its posture since April when it was first forced to publicly defend its decision not to discipline the swimmers, WADA said it had done nothing wrong and that its decision was supported by scientific and legal experts.

The escalation in the case represents perhaps the most significant investigation into a doping offense since the United States passed legislation known as the Rodchenkov Act in 2020, criminalizing doping in elite-level international sporting events wherever they take place. The investigation would be the first time U.S. authorities have focused on an international sports body under the act. WADA has been a critic of the new law since it was first proposed, arguing that criminalizing doping in one country would compromise WADA’s efforts to maintain one set of rules for sports everywhere.

In its statement, WADA raised questions about the United States involvement in investigating a matter that occurred beyond its borders.

“The public reports about this investigation validate the concerns expressed broadly by the international community about the passage of the Rodchenkov Act, under which the United States purports to exercise extraterritorial criminal jurisdiction over participants in the global antidoping system,” the statement said.

At the heart of the issue in the swimming case is the fact that the World Anti-Doping Agency agreed with China’s contention that the positive tests for the banned substance — a prescription heart medication — were the result of a “mass contamination event.” But some other antidoping experts and authorities found the claim highly dubious and charged that the lack of punishments and public identification of the athletes suggested a massive coverup.

In response, Congress in May called on the F.B.I. to open an investigation and congressional committees began their own investigations, including holding a hearing last week where Michael Phelps, the most decorated swimmer in Olympic history, testified about the need for accountability.

Antidoping officials at WADA have scrambled to protect the organization’s reputation and defended its handling of the case since news of the 23 cases was first made public. They have held numerous meetings with concerned groups, including hundreds of athletes and national antidoping bodies. Those efforts have failed to allay many of the concerns, and WADA is yet to publish any of the information it relied on to make its original decision.

A report from an independent prosecutor hired by WADA to investigate its decision making is set to be released before the Olympics. Even that might not be enough to stem the controversy before the start of the Games amid concerns over the independence of the Swiss official hired for the job.

Officials at World Swimming were keen to stress that they were asked to provide information as witnesses and were not being heard as targets of the federal investigation. WADA declined to comment.

The stakes and timing of an American investigation are particularly high given the country’s relationship with the International Olympic Committee. Los Angeles will host the next Summer Olympics after Paris, in 2028, while Salt Lake City was given preferred bidder status by the I.O.C. in bidding for the 2034 Winter Games.

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