World Aquatics confirmed on Thursday (just after midnight Friday AEST) that Brent Nowicki “was served with a witness subpoena by the United States government”.
In May, a House Committee on China asked the Justice Department and the FBI to launch an investigation under a federal law that allows investigations into suspected doping conspiracies even if they occurred outside the US.
The swimmers were allowed to compete at the previous Olympics in Tokyo despite testing positive for a banned heart medication.
Chinese officials blamed food contamination and the World Anti-Doping Agency accepted that explanation and has since defended its handling of the case.
Eleven of the swimmers are expected to compete for China at the Paris Olympics.
