Sitting at a witness table alongside three of her former gymnastics teammates, Simone Biles broke down in tears while explaining to a Senate committee that she doesn’t want any more young people to experience the kind of suffering she endured at the hands of Lawrence G. Nassar, the former national team doctor.

“To be clear, I blame Larry Nassar, but I also blame an entire system that enabled and perpetrated his abuse,” Ms. Biles, 24, said Wednesday as her mother, Nellie Biles, sat nearby, dabbing her eyes with a tissue.

Ms. Biles and hundreds of other girls and women — including a majority of the members of the 2012 and 2016 U.S. Olympic women’s gymnastics teams — were molested by Mr. Nassar, who is now serving what amounts to life in prison for multiple sex crimes. His serial molestation is at the center of one of the biggest child sex abuse cases in American history.

McKayla Maroney, an Olympian in 2012, also testified, describing in detail how Mr. Nassar repeatedly abused her, even at the London Games, where she won a gold medal. She said she survived a harrowing ordeal when she and Mr. Nassar were at a competition in Tokyo, certain she “was going to die that night because there was no way he was going to let me go.”

“That evening I was naked, completely alone, with him on top of me, molesting me for hours,” she said.

In 2015, when Ms. Maroney was 19 years old and before she had even told her mother what Mr. Nassar had done, she described her abuse to an F.B.I. agent during a three-hour phone call from the floor of her bedroom. When she finished, Ms. Maroney said the agent asked, “Is that all?” She said she felt crushed by the lack of empathy.

“Not only did the F.B.I. not report my abuse, but when they eventually documented my report 17 months later, they made entirely false claims about what I said,” Ms. Maroney testified. “They chose to lie about what I said and protect a serial child molester rather than protect not only me but countless others.”

The F.B.I. director, Christopher A. Wray, acknowledged the agency’s mishandling of the case and apologized to the victims. He said the F.B.I. had fired an agent who was involved in the case early — the one who interviewed Ms. Maroney. It was the first time anyone at the agency had submitted to public questioning about the F.B.I.’s failure to properly investigate a sexual abuse case that shook the sports world to its core.

Mr. Wray, who became the F.B.I. director in 2017 said he was “heartsick and furious” when he heard that the F.B.I. had made so many errors in the case before he took charge of the agency.

“I’m sorry that so many people let you down again and again,” Mr. Wray said to the victims. “I am especially sorry that there were people at the F.B.I. who had their own chance to stop this monster back in 2015 and failed, and that is inexcusable. It never should have happened, and we are doing everything in our power to make sure it never happens again.”

What horrendous experiences for these young women and what a colossal failure of the F.B.I.