Double world sprint champion Kelli White could be stripped of her two gold medals from Paris after testing positive for a stimulant.
The International Association of Athletics Federations, the sport's governing body, said White failed a doping test after the 100m - but passed a test after the 200m.
However, the IAAF said White's positive test after the 100 would be enough to disqualify her from the entire championships and cost her both medals.
"It doesn't change things," IAAF spokesman Nick Davies said. "We had a positive sample after the
100m - if we disqualify her, we disqualify her."
White, the first American to win both sprints at a world championship, could also face a two-year suspension.
White's positive sample evealed traces of the substance modafinil, which stimulates the nervous system and combats tiredness.
The substance is not on the banned list but is related to
drugs that are prohibited, and is likely to be forbidden at next year's Olympic Games.
"Modafinil is being observed as a possible substance for the
purpose of performance enhancement," said
IAAF vice president Arne Ljungqvist, the federation's
At a news conference on Saturday, White denied using any substance to enhance her performance.
She said she took medication - prescribed by her doctor, Brian Goldman of San Francisco - for a tiredness disorder called narcolepsy.
"I know I that I did nothing wrong and sought no advantage over my competitors," she said in a statement. "I am confident that things will work out in the end.
"The mere fact of this allegation is personally harmful and hurtful," she added. "I have never taken any substance to enhance my performance."
She said she didn't seek a medical waiver or list the
product on her doping control form because she had "no
idea" it contained a banned substance.
"Given that it was not on the banned list, I think it is
understandable why I didn't realize that I needed to
declare it," White said.
IAAF general secretary Istvan Gyulai said the indications pointed towards the matter being ruled as a doping case, which would result in White's disqualification from the championships.
Ljungqvist said the IAAF told USA Track & Field to hold a hearing and take any disciplinary action, with the decision subject to final approval by the world body.
The outcome is unlikely to be decided before the end of
the championships on Sunday.
White did not run for the USA in the women's 4x100m relay final on Saturday night where they were surprisingly beaten by France.
She previously served a six-month ban in France after a sample she provided at a meeting at Saint Denis in July 2002 showed traces of corticoid.
Corticoid is an anti-inflammatory steroid and White lacked a medical
certificate, but the IAAF cleared her of any wrongdoing.