Two key American athlete groups sent a letter to the country's drug czar calling for a "truly independent" investigation of the case that led to 23 Chinese swimmers not being sanctioned despite testing positive for a banned heart medication.

The USA Swimming Athletes' Advisory Council and the Team USA Athletes' Commission sent the letter Monday to Rahul Gupta, the director of the federal government's Office of National Drug Control Policy.

WORLD ANTI-DOPING AGENCY SAYS WHY CHINESE SWIMMERS WERE CLEARED DESPITE POSITIVE TESTS FOR BANNED SUBSTANCE

Gupta is on the World Anti-Doping Agency's executive committee and is expected to take part in a special meeting Tuesday with other government officials in the Americas to discuss the doping case.

His role is important because governments fund half of WADA's annual budget; the U.S. has held up payments in the past when it felt WADA wasn't following through on its mission.

"As athletes, we have to trust WADA to set and enforce standards that will ensure fair play and protect our rights," the athletes wrote to Gupta. "WADA’s failure to follow its own rules and procedures in the wake of the positive tests of these 23 Chinese athletes has broken this trust."

WADA has strongly denied that any rules were broken, saying Chinese anti-doping authorities ruled the swimmers' samples from January 2021 had been contaminated and there was no effective way for the world's top anti-doping watchdog to appeal that ruling. Some of those swimmers went on to compete at the Tokyo Olympics later that year.

Last week, WADA announced it was appointing a veteran Swiss prosecutor, Eric Cottier, to review how it handled the cases. But that decision was criticized both because the prosecutor comes from the same Swiss canton (state) as where the International Olympics Committee and World Aquatics are located, and also for what critics say is a limited scope of review.

After a scheduled meeting of government leaders from the Western Hemisphere last week, Gupta put out a statement calling WADA's appointment of Cottier "an important first step in addressing the recent doping allegations."

Gupta's office did not immediately return an email from The Associated Press seeking comment about Monday's letter.

Among those on the swimming athletes' council is Lilly King, who has long been outspoken about doping in her sport.

"Once again, we are heading into another Olympic and Paralympic games with serious concerns about whether the playing field is level and the competition fair," the athletes wrote to Gupta.