Travis Tygart, head of the United States Anti-Doping Agency (USADA) said on Monday he had doubts about the effectiveness of an investigation into positive drug tests by 23 Chinese swimmers.

The World Anti-Doping Agency (WADA) has faced mounting pressure to explain how Chinese swimmers tested positive for a banned substance weeks before the Tokyo Olympics but escaped punishment with some going on to win gold medals.

A report by Swiss prosecutor Eric Cottier looking at if proper procedures were followed by anti-doping and swimming officials is expected to be made public on Tuesday but Tygart was not waiting for the findings calling the investigation, "more of a self-serving check the box type of exercise".

"We were ultimately glad that WADA was forced to have an independent review," said Tygart in a video message to U.S. athletes. "We, of course, were disappointed that the very staff whose decisions in this process were at question were the ones to set the terms of reference for this review.

"We have seen in anti-doping and other sport movement situations whitewash type reports when they hire these independent people."

WADA has conceded Chinese doping officials could have handled the matter better but has vigorously defended its handling of the case saying it followed rules laid out in the anti-doping code and has no authority to impose provisional suspensions.

The New York Times reported in April that the 23 Chinese swimmers tested positive for trimetazidine (TMZ), a medication that increases blood flow to the heart and is used to treat angina.

The swimmers were cleared by a Chinese investigation which said they were inadvertently exposed to the drug through contamination. The report determined the swimmers were staying at a hotel where traces of TMZ were discovered in the kitchen.

WADA said it had no evidence to challenge China's findings and that external counsel had advised against appealing them.

While Tygart remains sceptical, the findings of Cottier's report could ease some of the criticism on WADA, who confirmed last week that it was aware the matter is being investigated by U.S. law enforcement.

A U.S. House of Representatives committee in May called on the Department of Justice (DOJ) to launch inquiries ahead of this year's Paris Olympics into the Chinese doping cases.

Tygart said, that if the Cottier report does not provide answers that a U.S. backed investigation will and called for those found responsible to be held accountable.

"Days ago, the international federation for swimming acknowledged a federal law enforcement investigation into this situation," said Tygart.

"Let's hope that if this WADA review doesn't get to the bottom of it that then that reported (US) investigation will ultimately hold the organisations accountable to the extent that they can."