World 400m champion Jerome Young failed a doping test in Paris last month, according to a French newspaper.
L'Equipe reported that the 27-year-old tested positive for the banned blood booster EPO on 23 July.
The news follows a previous test in 1999 when his blood was found to contain traces of nandrolone, though he was later exonerated of all charges.
He then won gold in the 400m relay at the 2000 Olympics - a result that could now be reversed.
Young's previous failed drug test was not revealed until last year.
After it came to light, the International Association of Athletics Federations ruled last month that the US 400m relay team should be stripped of their Sydney gold.
The International Olympic Committee executive board is scheduled to meet before the Olympics start on 13 August, and must decide whether or not the medals should be taken back.
Young will not run in Athens as he failed to make the US team.