Editor's Note: "You had one job. You literally had one job, and you couldn't protect us!" Olympic gold medal gymnast Simone Biles told reporters earlier this week.

Biles, in Kansas City for the U.S. Gymnastics' Championship, said that USA Gymnastics failed to protect her and other athletes from Larry Nassar, the former team doctor. Nassar is now in prison serving time for numerous counts of sexual assault.

"It's not easy being out here," Biles said. "I feel every day is a reminder of what I went through."

60 Minutes first reported on the allegations against Nassar in February 2017, when three former gymnasts -- Jamie Dantzscher, Jessica Howard and Jeanette Antolin -- told correspondent Dr. Jon LaPook about the abuse they endured during Nassar's "treatments."

"I remember thinking something was off, but I didn't feel like I was able to say anything because he was, you know, this very high-profile doctor," Howard told LaPook.

Biles disclosed in 2018 that she too had been assaulted by Nassar.

The U.S. women's gymnastics team – for all its success over the past few decades – has become embroiled in a dark and disturbing scandal concerning sexual abuse. Last year, the Indianapolis Star investigated cases in which male coaches, members of the national governing organization USA Gymnastics, were accused of sexually abusing female gymnasts. That report prompted young women to come forward with accounts of abuse they had suffered within the U.S. gymnastics system for many years as young girls and competitive gymnasts. These new accusations concern not a coach, but a prominent doctor who'd been working with U.S. Olympic and national teams and other athletes for three decades.

Former Team USA gymnasts, from left: Jamie Dantzscher, Jessica Howard and Jeanette Antolin CBS News

More than 60 women have filed complaints so far, and some believe that number may reach into the hundreds. Now, for the first time, three former members of U.S. national teams, one an Olympic medalist, describe – in what you should be warned is disturbing detail –the treatment they received from Dr. Lawrence Nassar -- a man they trusted and felt so comfortable with, they called him, "Larry."

Jeanette Antolin: All the girls liked Larry.

Jamie Dantzscher: He was, like, my buddy. He was on my side.

Dr. Lawrence Nassar

Jessica Howard: He was so sure of himself. And as a young girl, you're confused. You don't know what's going on.

Jessica Howard was the U.S. national champion in rhythmic gymnastics from 1999 to 2001.

Jeanette Antolin competed with the U.S. national team from 1995 to 2000.

She helped UCLA win three national championships.

Jamie Dantzscher won a bronze medal in the 2000 Olympics and was recently inducted into UCLA's Athletic Hall of Fame.

They were teenagers, in a sport where injuries are common, and the professional they turned to for help staying in competition was this man -- seen here in instructional videos he posted on his web site. Lawrence Nassar, an osteopathic physician, was one of the most famous doctors in the world of gymnastics. As a trainer and doctor he worked with Olympic and national womens' artistic gymnastics teams for more than two decades. That's him right after Kerri Strug's famous ankle injury in the 1996 Olympics in Atlanta.

And that's him today. Since December, he's been held without bail in Michigan, where he worked at Michigan State University's sports medicine clinic. He's charged with possession of child pornography and criminal sexual conduct involving the daughter of a family friend. Investigators were able to make the case against him because gymnasts went public after years of silence. The police and FBI are now investigating dozens of other cases involving Nassar -- some decades old, others within the last two years.

Jamie Dantzscher CBS News

Jamie Dantzscher says she started seeing Dr. Nassar around 1995, after she became a member of the U.S. junior national team.

Jamie Dantzscher: I started having really bad lower back pain on my right side on my back. So I went to him for my back pain.

Jon LaPook: What specifically would he do?

Jamie Dantzscher: He would put his fingers inside of me and move my leg around. He would tell me I was going to feel a pop. And that that would put my hips back and help my back pain.

Jon LaPook: How old were you then when he first did that procedure?

Jamie Dantzscher: I was either 13 or 14.

Jessica Howard CBS News

Jessica Howard: I was 15 years old and I had a hip problem. A very severe hip problem. And USA Gymnastics suggested that I go to the Karolyi Ranch to work with their doctor.

The Karolyi ranch outside Houston, Texas, is a mecca for elite gymnasts who have given up any semblance of normal childhood to pursue their Olympic dreams. Run by the legendary coaches Bela and Martha Karolyi, it's where members of the U.S. national team for artistic gymnastics come roughly once a month for several days of intensive training. The girls stayed in cabins on the property, and Dr. Nassar would be there to provide medical treatment.

Jessica Howard: He started massaging me. And-- he had asked me not to wear any underwear. And then he just continued to go into more and more intimate places.

Jon LaPook: And when that happened, what, what was going through your head?

Jessica Howard: I remember thinking something was off but I didn't feel like I was able to say anything because he was, you know, this very high-profile doctor. And I was very lucky to be at the ranch working with him.

Jon LaPook: Did any of the other girls in your cabin talk to you about Dr. Nassar?

Jeanette Antolin CBS News

Jessica Howard: Yes. The girls would say yeah he touches you funny.

Jeanette Antolin: I remember being uncomfortable because of the area. But-- in my mind, I was like, "If this helps, I'll do anything."

Jon LaPook: Did you ever complain to anybody about it?

Jeanette Antolin: No.

Jon LaPook: Why not?

Jeanette Antolin: It was treatment. You don't complain about treatment.

Dr. Nassar has pled not guilty to the charges against him in Michigan. In a statement from his lawyers, he has defended his treatment as legitimate. There is a rare therapy for back and hip pain where specialists massage areas inside the vagina. But for a minor, it's expected such a procedure should involve a chaperone and use of a glove.

Jon LaPook: Did he use a glove?

Jamie Dantzscher: No.

Jon LaPook: And how many times did you have this kind of a procedure?

Former Team USA gymnasts speak with Dr. Jon LaPook, CBS News' chief medical correspondent CBS News

Jamie Dantzscher: I mean, it happened all the way to the Olympics in Sydney, till I was 18.

Jon LaPook: From the time you were around 13 or so until 18?

Jamie Dantzscher: Yes.

Jon LaPook: And it was just-- in your mind, normal medical treatment?

[Jamie makes expression]

John Manly: You've got a 52-year-old man placing his hand in the vagina of nine-year-olds ungloved for no good reason. Wrong.

California attorney John Manly represents the women we interviewed and more than 40 others – one as young as 9 years-old, and most under 18 at the time they say they were abused.

Jon LaPook: How many women do you think he did that to?

John Manly: We know there are at least 60 that have come forward. But my best estimate is it's in the hundreds and possibly more.

Jon LaPook: Are you saying that members of the last two Olympic teams from Rio and from London were affected by Dr. Nassar? That they were abused by him?

John Manly: I believe what-- at the end of the day there are members of every single Olympic team since 1996 he did this to. That's what we're gonna end up with.

Jon LaPook: What makes you so sure about that?

John Manly: Because this is somebody who is a serial predator. But the story here is that no one was watching to protect these girls. And they put medals and money first.

By "they," Manly means USA Gymnastics and the Karolyis. He's not arguing they knew anything about sexual abuse. Many years went by before the women we interviewed complained to anyone in authority. But part of the reason for that, Manly argues, was a high-pressure, emotionally abusive environment at the ranch, which he says made it easy for Nassar to win the girls' trust.

Jamie Dantzscher: I mean, the-- like, yelling and screaming, that was, like, normal.

Jon LaPook: Really?

Jamie Dantzscher: Yeah.

Jon LaPook: What kind of abusive things were said to you?

Jamie Dantzscher: It was never good enough. "You're not good enough."

Jeanette Antolin: the pressure that they put on you to-- be perfection for them, it was very overwhelming and stressful.

John Manly: it was an environment of fear. And he stepped in and became the good guy. And—

Jon LaPook: Dr. Nassar did?

John Manly: Dr. Nassar did. And he gave 'em candy. He gave 'em encouragement. He acted like he cared about them. No one else there gave that impression.

Jon LaPook: What were these girls so afraid of?

John Manly: Not being able to fulfill their dream. I mean you've given up your childhood and you've given up your adolescence to represent your country. And the Karolyis and the selection team who are there have control on who goes. So your fate is in their hands. You must do what they say.

On behalf of the women, attorney Manly is suing the Karolyis and USA Gymnastics for failing to protect their athletes. USA Gymnastics president Steve Penny declined to speak with us on camera about Dr. Nassar. In a statement, the organization said it is "appalled that anyone would exploit a young athlete or child in this manner." USA Gymnastics "first learned of an athlete's concern about Dr. Nassar in June 2015," the statement said. Five weeks later, after an internal review, it "reported him to the FBI and relieved him of any further assignments." USA Gymnastics told us it has long had a policy that adult staff should "avoid being alone with a minor."

Jon LaPook: How often were you alone with him?

Jeanette Antolin: Most of the time.

Jon LaPook: Just in the treatment area, or also in your bedroom?

Jeanette Antolin: In our cabins. They were like cabins. Yeah.

Jon LaPook: That's like your bedroom.

Jeanette Antolin: Yeah. Uh-huh (affirm).

Jon LaPook: Yeah. And did the Karolyis know that Dr. Nassar was alone with you for these treatments?

Jamie Dantzscher: Yeah.

Jon LaPook: How-- how do you know that?

Jamie Dantzscher: Well, they had to know. I mean, there-- there was no one else sent with him. And that's the thing, too, to think, like-- what-- they-- in-- in the bed? Why would you-- like, the treatment was in the bed, in my bed that I slept on at the ranch.

Bela and Martha Karolyi declined to give us an interview, but in a statement they said they "were never aware" that Nassar was performing this procedure or was "visiting athletes in their rooms without supervision." They also deny that there was an emotionally abusive environment at the ranch.

Long before Dr. Nassar's arrest late last year, USA Gymnastics was facing criticism over its handling of sexual abuse complaints about coaches at its member gyms throughout the country. According to an investigation published by the IndyStar in August, USA Gymnastics received a complaint that one of its coaches, William McCabe, should be locked up "before someone is raped," but did not report it to the authorities at the time. It was only after the mother of a gymnast called the FBI seven years later that McCabe was sentenced to 30 years in prison for sexually exploiting gymnasts. Marvin Sharp was named USA Gymnastics women's coach of the year in 2010, but was the subject of a sexual abuse complaint the following year.

USA Gymnastics didn't report Sharp to the police until four years later when another complaint came in. Sharp killed himself in jail while facing molestation and child pornography charges.

Dianne Feinstein: An association has a responsibility, or should have a responsibility. And that is to take care of its members.

Jon LaPook: And do you think USA Gymnastics has done that?

Dianne Feinstein: No.

Senator Dianne Feinstein is the ranking member of the Senate Judiciary Committee. She's met with the women we interviewed and other gymnasts and is now working on legislation to correct what she sees as a problem in the reporting of sexual abuse complaints.

Dianne Feinstein: If an amateur athletic association, like USA Gymnastics, receives a complaint, an allegation, they must report it right away to local police and the United States attorney.

Jon LaPook: So this wouldn't apply just to gymnastics. It would apply to all Olympic sports that have a national governing body?

Dianne Feinstein: All amateur athletic organizations. That's right.

It's been nearly two decades since the women we interviewed competed at the highest level of their sport.

Today, they say they're still grappling with the psychological impact of their competitive careers. Jeanette Antolin told us it was only last year, after speaking with other gymnasts, that she realized Dr. Nassar hadn't been helping her with her back pain after all.

Jeanette Antolin: It was like -- almost like a light bulb went off. Like, "Oh my gosh. Like-- are you kidding me? Like-- I trusted this man." And just knowing how vulnerable I was as a kid, to even not even think that something like that would be inappropriate, just ruined me.