Simone Biles, seen by many as the foremost gymnast of all time, was one of four who testified before the Senate.
The other three were McKayla Maroney, Maggie Nichols and Aly Raisman.
Biles was emotional in the courtroom and accused the FBI of turning a blind eye to Nassar's abuse.
She also said that the American Gymnastics Federation and the American Olympic Committee "knew that I had been abused by their national team doctors long before I learned that they knew about it."
The scandal surrounding Nassar was unfolded several years ago, when he was accused by more and more gymnasts of sexual abuse.
He has been serving a long prison sentence since 2018.
Chris Wray, the FBI's chief of staff, and the Justice Department's investigator Michael Horowitz, who is behind a report criticizing the FBI's actions, will be heard later.
CLIP: Biden pays tribute to Biles: "A role model for everyone"
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Joe Biden is impressed with Simone Bile's achievement.
Photo: TT News Agency