The DLV announced on Monday it was investigating Schumann, who won the 800m gold in Sydney in 2000, and former 400m European champion Grit Breuer for suspected violation of doping rules.
"I've always said that I never went beyond the bounds of what's allowed and that has not changed to this day," Schumann said in a statement.
"I've already directly stated that I will be helpful in every way possible, to clear things up, support the DLV and also to document that I have nothing to hide."
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The DLV said in a statement on Wednesday it was also investigating a third athlete, Ulrike Urbansky.
The sport's governing body in Germany is basing its work on evidence that arose during the case of Thomas Springstein.
Springstein, who has in the past coached both athletes, was given a 16-month suspended sentence in March this year after he was found guilty of giving banned drugs to an under-age athlete.
Breuer, the former partner of Springstein, was banned for three years from 1992 to 1995 for taking the stimulant Clenbuterol.
In 2004 she was accused of skipping a drugs test in South Africa but was later cleared.
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