The German Olympic Sports Confederation (DOSB) said it had been informed by the International Olympic Committee (IOC) that the "A" sample "of a member of the German Olympic team produced a result that diverged from the norm".
The "B" sample is to be tested on Friday while the IOC's disciplinary commission will meet to discuss the issue, the DOSB said in a statement on its website.
There were no details on the name or sport of the athlete, or whether the test had been carried out at the Games or before.
The IOC confirmed the content of the German statement but said no further information would be given until the investigation is over.
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