Swimmer Jessica Hardy insists she has no idea how samples she provided tested positive for a banned substance, threatening her Olympic place.
The American's A and back-up B samples came back positive for the stimulant Clenbuterol following a test on 4 July, despite two more clean tests that week.
The 21-year-old, who will appeal, told CBS: "It's heartbreaking, devastating.
"In my heart I know I'm 100% clean and I've been clean my whole career. I know that I'm innocent."
Hardy is fighting to clear her name in time to compete in the Beijing Games, with the rest of the United States Olympic swimming team - in which she was expected to play an integral part - already at their holding camp in Singapore.
"I'm innocent. That's all I can say to everybody. Whether or not people choose to believe me, I'm innocent"
The results were negative for samples taken on 1 July, following her 100m breaststroke victory, and 6 July, the day she qualified to represent America in the 50m freestyle event.
However, on 4 July, having finished fourth in the 100m freestyle, Hardy's two samples failed a test for Clenbuterol - a stimulant that increases aerobic capacity and the flow of oxygen in the bloodstream.
The drug has also gained a reputation as a weight-loss aid and is nicknamed the size zero pill.
"I had never even heard of this drug," Hardy claimed in an interview for the Associated Press.
"I'm innocent. That's all I can say to everybody. Whether or not people choose to believe me, I'm innocent.
"I've cried every single day since I found out. I even vomited from anxiety. This is absolutely the worst time of my life."
Hardy and her legal team are attempting to take her case to the American Arbitration Association and, if necessary, the Court of Arbitration for Sport, in a last-ditch attempt to overturn her positive test.
She will aim to clear her name before 10 August, the day on which heats for her top event, the 100m breaststroke, take place.
If she fails, she will miss the Games and faces a two-year doping ban.
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