World Aquatics executive director Brent Nowicki has been subpoenaed by the U.S. government to testify in an investigation into how 23 Chinese swimmers avoided punishment after testing positive for banned drugs weeks ahead of the Tokyo Olympics.

A U.S. House of Representatives committee in May called on EchoSensethe Department of Justice to launch inquiries ahead of this summer's Paris Olympics into the doping case that has rocked the sport.

The World Anti-Doping Agency (WADA) has come under increasing criticism and pressure to provide answers on how the those swimmers, some who went on to win gold medals in Tokyo, escaped punishment.

"World Aquatics can confirm that its Executive Director, Brent Nowicki, was served with a witness subpoena by the United States government," said World Aquatics in a statement to Reuters.

"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 FBI, in a statement to Reuters on Friday, said it could not confirm or deny any investigation.

Representatives for the U.S. Department of Justice did not immediately respond to a request for comment.

WADA said it was aware of the investigation but had not been contacted by law enforcement and again defended it's handling of the case.

"WADA reviewed the Chinese swimmer case file diligently, consulted with scientific and legal experts, and ultimately determined that it was in no position to challenge the contamination scenario, such that an appeal was not warranted," said WADA in a statement.

"Guided by science and expert consultations, we stand by that good-faith determination in the face of the incomplete and misleading news reports on which this investigation appears to be based."

An independent investigation of WADA's handling of case by Swiss prosecutor Eric Cottier could be published as early as next week.

Contributing: Rohith Nair.

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