Double world sprint champion Kelli White could lose both her gold medals after testing positive for a stimulant following Sunday's 100 metres women's final.
Although cleared to run in Saturday's 4x100 metres final, White was taken off the team that finished second to France.
White, the first American to win the 100-200 sprint double, tested positive for modafinil, which is at present not on the International Association of Athletics Federations' (IAAF) banned list.
IAAF vice-president Arne Ljungqvist told a news conference White could still be disqualified because modafinil was clearly related to similar drugs that are banned, which means she can still be sanctioned under the federation's rules.
Ljungqvist confirmed modafinil, which is used to treat loss of concentration or sleepiness, would be placed on the World Anti-Doping Agency banned list next year. "Modafinil is being observed as a possible substance for the purpose of performance enhancement," he said
He added that White had neither declared she was taking the drug nor sought a prior exemption.
He said further investigation was needed to see whether it was related to stimulants such as ephedrine, for which the punishment is an official warning and disqualification, or more serious drugs such as amphetamines.
Athletes testing positive for amphetamines are disqualified and banned for two years.
White, reading from a prepared statement, told a news conference she had never taken any performance-enhancing substance.
"The mere fact of this allegation is personally harmful and hurtful," she said. "I have worked very hard for the medals I earned this week and I'm going to work very hard to keep those."
She said close members of her family had been under treatment for narcolepsy for years and she had been prescribed modafinil.
"It has improved the condition in my day-to-day life and helped me function normally," she said.
White said she had not sought an exemption or entered it on her doping control form because it was not on the banned list.
"Given that it was not on the banned list, I think it is understandable why I didn't realise that I needed to declare it on my doping form," she said.
White (26) was banned from competing in France from January 1 to June 30 this year after testing positive for triamcinolone at last year's Paris Golden League meeting.
Triamcinolone, a drug used to combat asthma, is illegal in France although it is not on the IAAF banned list.
