Australian swimmer Shayna Jack tested positive for a banned substance before the World Aquatics Championships, Swimming Australia has said.
Jack, 20, who withdrew from the championships on 14 July, says she “did not take the substance knowingly”.
The Australian Sports Anti-Doping Authority found an adverse result in an out-of-competition test on 26 June.
Swimming Australia said it then provisionally suspended Jack and she was unable to compete in South Korea.
The Asada investigations are ongoing.
“As you would expect we are bitterly disappointed with allegations a swimmer has a prohibited substance in her system, although it is important to point out that the matter is yet to be determined,” said Swimming Australia boss Leigh Russell.
In a post on social media, Jack, who originally withdrew from the World Championships for “personal reasons”, said: “Swimming has been my passion since I was 10 years old and I would never intentionally take a banned substance that would disrespect my sport and jeopardise my career.
The four-time World Championship medallist added: “It is with great sadness and heartache that I had to leave due to allegations of having a prohibited substance in my system. I did not take this substance knowingly.
“Now there is an ongoing investigation and my team and I are doing everything we can to find out when and how this substance has come into contact with my body.”
Russell said the governing body would “provide appropriate support for Shayna”.
She added that Swimming Australia has “consistently supported a zero tolerance approach to doping”.
Earlier in the Championships, Jack’s Australian team-mate Mack Horton refused to share the podium with China’s controversial gold medallist Sun Yang after the 400m freestyle.
Sun served a three-month ban in 2014 after testing positive for banned stimulant trimetazidine,
Britain’s Duncan Scott also refused to join Sun on the podium after winning bronze in the 200m freestyle, behind the Chinese.