Aly Raisman is one of the most accomplished American gymnasts of all time. She's won six Olympic medals, three of them gold, and was captain of the U.S. teams that dominated the last two summer games: in London in 2012, and Rio in 2016. Doing a backflip on a balance beam takes a certain type of courage. Last year, Raisman displayed courage of another kind when she spoke with us for the first time about a deeply personal and painful part of her life. It was a stunning development for USA Gymnastics, the national governing body for her sport: Raisman said she was sexually abused while competing on the U.S. National team. Her story, and those of other women, set off an extraordinary chain of events that led to the resignations of the most powerful people in her sport, including just this past week, the current president of USA Gymnastics, Kerry Perry. But it all began with Raisman and hundreds of other athletes speaking out about abuse that Raisman says was difficult for her to acknowledge.

Aly Raisman CBS News

Aly Raisman: I was in denial. I was like, "I don't thi-- I d-- I don't even know what to think." It-- you don't wanna let yourself believe but, you know, I am-- I am-- I am a victim of-- of sexual abuse. Like, it's really not an easy thing to let yourself believe that.

Jon LaPook: You're saying you were sexually abused.

Aly Raisman: Yes. Absolutely.

Jon LaPook: By the National Team doctor.

Aly Raisman: Yes.

Jon LaPook: While you were out there representing your country.

Aly Raisman: Yes.

"I didn't know anything differently. We were told he is the best doctor. He's the United States Olympic doctor and the USA Gymnastics doctor, and we were very lucky we were able to see him."

Few athletes have represented their country with as much distinction as Aly Raisman.

Her floor routines at the last two Olympics dazzled audiences and judges and provided some of the most iconic moments of the Summer Games.

The doctor she says abused her, Larry Nassar, worked with the U.S. Women's National Team and athletes at Michigan State University for more than two decades. Raisman says Nassar first treated her over eight years ago, when she was 15 years-old.

Aly Raisman: I was just really innocent. I didn't really know. You know, you don't think that of someone. You know, so I just-- I trusted him.

Jon LaPook: You thought it was medical treatment.

Aly Raisman: I didn't know anything differently. We were told he is the best doctor. He's the United States Olympic doctor and the USA Gymnastics doctor, and we were very lucky we were able to see him.

Larry Nassar in court. CBS News

Nassar is now in prison. He pleaded guilty to possessing child pornography and to sexually assaulting underage girls, most of them athletes who say his treatment for hip and back pain involved putting an ungloved hand inside the vagina.

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

The first top gymnasts to speak out about Nassar appeared on 60 Minutes in February 2017, describing what he did to them more than a decade before Raisman joined the national team. Over 400 women have alleged similar abuse. Last year, Nassar and his lawyers declined to comment for this story.

Raisman, now 24, talks about her experiences in a new book called "Fierce." It's the story of a girl who dreamed of going to the Olympics, and how she managed to get there. Raisman says she will not discuss the graphic details of what Nassar did to her, but she does provide new insight into a scandal that goes to the highest level of her sport. She told us a lot of people have asked her why Nassar's accusers didn't speak up sooner.

Aly Raisman: Why are we looking at why didn't the girls speak up? Why not look at what about the culture? What did USA Gymnastics do, and Larry Nassar do, to manipulate these girls so much that they are so afraid to speak up?

Jon LaPook: You're angry.

Aly Raisman: I am angry. I'm really upset because it's been-- I care a lot, you know, when I see these young girls that come up to me, and they ask for pictures or autographs, whatever it is, I just-- I can't-- every time I look at them, every time I see them smiling, I just think-- I just want to create change so that they never, ever have to go through this.

About 165,000 athletes and 3,400 gyms are members of USA Gymnastics. Raisman is calling for major changes in personnel, training, and education to keep athletes safe.

Raisman first joined USA Gymnastics when she was in elementary school. As she got older, she sacrificed family vacations, parties and boyfriends, in favor of grueling workouts, four to seven hours a day, at a gym in suburban Boston run by her trusted coach Mihai Brestyan, who she credits for much of her success.

Jon LaPook: Coming up in gymnastics did you think of yourself as naturally very talented?

Aly Raisman: No. It's funny because Mihai will say I'm the most uncoordinated Olympian in the whole world.

Jon LaPook: Is it true that you're afraid of heights?

Aly Raisman: Yes. I'm very afraid of heights.

Jon LaPook: How is that possible? You're flying through the air.

Aly Raisman: I don't know. You know, it doesn't scare me on floor. But it scares me on bars.

Raisman's parents CBS News

Her hard work and dedication landed her a spot on the U.S. team for the London Olympics in 2012. During the qualifying round in London, a video of her parents, Lynn and Rick Raisman, went viral.

Jon LaPook: At the London Olympics, you became perhaps the most famous cheering section in history.

Rick Raisman: I just think we got caught up in the moment. The amount of pressure and, you know, that she was under at that time, it's just not easy for us to watch.

In the first key test in London -- the team finals -- Aly's performance on the floor clinched gold for the U.S.. Their first gold ever on foreign soil.

"I didn't know the signs. I didn't know what sexual abuse really was. And I think that needs to be communicated to all of these athletes, no matter the age."

It was a picture-post-card moment that only told part of the story. One of the women standing next to Raisman, her teammate, McKayla Maroney, has said that Nassar sexually abused her before the team's victory. In a Twitter post, Maroney wrote that Nassar's abuse, "started when I was 13 years old, and it didn't end until I left the sport…" Raisman says she and other athletes did not realize at the time that Nassar gained their trust through a predatory technique designed to build an emotional bond with the athletes. It's called "grooming."

Aly Raisman: He would always bring me, you know, desserts or gifts. He would buy me little things. So I really thought he was a nice person. I really thought he was looking out for me. That's why I want to do this interview. That's why I wanna talk about it. I want people to know just because someone is nice to you and just because everyone is saying they're the best person, it does not make it okay for them to ever make you uncomfortable. Ever.

USA Gymnastics has a long-standing policy that adults should "avoid being alone with a minor." Despite that policy, Raisman says she was alone with Dr. Nassar. He treated her and other athletes in their hotel rooms during competitions abroad.

Jon LaPook: All those years, you were out there representing the United States of America. Did our country's sports system look out for you the way it should have?

Aly Raisman: No. I think-- Nobody ever educated me on, "Make sure you're not alone with an adult." You know, "Make sure he's not making you uncomfortable." I didn't know the signs. I didn't know what sexual abuse really was. And I think that needs to be communicated to all of these athletes, no matter the age.

After helping her team win gold at the London Olympics, Raisman had a crisis of confidence in the next competition -- the individual all-around. She finished fourth. By the time she got to her last chance for individual gold -- the floor exercise final -- she was so nervous, she considered not doing one of her more difficult moves.

Jon LaPook: But then your coach said something to you.

Aly Raisman: He said, "You worked too hard to not be Olympic champion because you're a little afraid."

She took Coach Mihai's advice and won gold.

Aly Raisman: It felt like I was like floating. It felt like it was effortless. And I've never felt like that before. But it was, like, the best feeling in the world… I learned a valuable lesson that day. In the all-around final, I was nervous. I doubted myself, and I made a mistake. In the floor final, I knew I was gonna hit the best routine of my life, and I did.

Aly Raisman of the U.S. competes on the balance beam in the Gymnastics Women's Team Final at the Rio Olympic Games, Aug. 9, 2016. Mike Blake/REUTERS

She returned from London with three medals, a hometown hero, and an international celebrity.

She competed on Dancing With the Stars, got involved in business ventures and struck endorsement deals with Reebok and other companies. But it wasn't enough. She wanted to compete in a second Olympics -- something no American gymnast had done in 16 years.

Jon LaPook: Did you think that some people had written you off

Aly Raisman: Yes, absolutely. I remember when I first said I was coming back some other coaches would just say, "I think it's gonna be really hard for you. I don't think you can do it."

Jon LaPook: That's not the thing to say to you.

Aly Raisman: Yes, yeah--

Jon LaPook: "I don't think you can do it." Right?

Aly Raisman: Yes. but There were so many days where I just thought, "Should I stop? Should I, this is just crazy. This is so hard."

It got even harder in the summer of 2015 -- a year before the Rio Games -- when an investigator hired by USA Gymnastics paid Raisman a visit. A coach had raised concerns about Dr. Nassar's treatment of athletes. Raisman says she was caught off guard when the investigator asked her about it.

Aly Raisman: And I said, you know, "Well, he-- his touching makes me uncomfortable, but he's so nice to me. And I-- I don't think he does it on purpose because, you know, I think he cares about me."

Jon LaPook: So it was only after the investigator left that you began to put the pieces together.

Aly Raisman: Yeah. I mean, I think it's important for people to know too I'm still trying to put the pieces together today. You know it impacts you for the rest of your life.

"Predators aren't just strangers. They can be highly educated. They can be very well-respected in the community. It could be a family member, it could be a family friend."

After the Nassar scandal broke, USA Gymnastics president Steve Penny resigned. His replacement was Kerry Perry, who stepped down this past week. In a statement last year, USA Gymnastics told us it had adopted a "safe sport policy" that requires "mandatory reporting" of suspicions of sexual abuse, and sets standards to "prohibit grooming behavior" and "prevent inappropriate interaction" between athletes and adults. "USA Gymnastics is very sorry that any athlete has been harmed…" the statement said, "…we want to work with Aly and all interested athletes to keep athletes safe."

But last year the Raismans told us the changes hadn't gone far enough.

Lynn Raisman: With the exception of Steve Penny, the same people [are] in place. So I don't really have tremendous hopes that a lot of those policies will be enforced.

Jon LaPook: A lotta people are asking, where were the parents?

Lynn Raisman: We were there. But if she's not knowing that it's wrong -- never in a million years did I ever even think to say, "Hey, when you see the team doctor, is there someone with you?"

Jon LaPook: If you could hit the rewind button, is there anything you would have done differently?

Lynn Raisman: I think the most important thing, if anyone takes anything away from this interview is sit down with your kids and explain to them that predators aren't just strangers. They can be highly educated. They can be very well-respected in the community. It could be a family member, it could be a family friend. So, you know, that's really, the, I mean, if I could go back in time, I would do that.

Aly Raisman CBS News

Aly Raisman's experiences with Dr. Nassar haunted her, but did not stop her from pulling off one of the most memorable performances of the Olympic Games in Rio.

In the all-around, she won silver.

Aly Raisman: I finally competed in the all-around final without any major error. I finally competed for myself. And I finally believed in myself.

Aly Raisman: That was just such a good feeling it was really empowering for me.

After she returned from Rio, she says she spoke with FBI agents who had opened an investigation of Larry Nassar.

Last summer, Raisman and her teammates from Rio were inducted into the USA Gymnastics Hall of Fame.

Aly Raisman: We must remember that protecting athletes comes first, and doing right by athletes is always the priority. I love our sport so much, and I want the best for it.

Some of the most powerful people in the sport were there. But Raisman says it seemed that many of them were giving her the cold shoulder.

Aly Raisman: There was a table of a lot of people that are very high up in USA Gymnastics that were in the room. And they didn't come over. You know, my teammates and I were all sitting at the table, and they did not come over to say hi to us or to congratulate us.

Jon LaPook: After your speech, or--

Aly Raisman: Before, at all. The whole time. All we've done is worked really hard. We love the sport. And we were treated like, you know, "We don't want anything to do with you girls."

Jon LaPook: Are you concerned that being so outspoken could jeopardize your odds of making the next Olympics?

Aly Raisman: You know, I think that's a very valid point. But I think that this, speaking out, and creating positive change so that athletes are safe, is more important than any Olympic medal you could ever win.

Since we first broadcast this story, Aly Raisman has emerged as a leader in the fight against sexual abuse, and has not been training for the Tokyo Olympics in 2020. At a court hearing in Michigan in January, Raisman was one of more than 150 women who confronted Larry Nassar.

Aly Raisman: Imagine feeling like you have no power and no voice. Well, you know what, Larry? I have both power and voice and I am only beginning to just use them.

Nassar was sentenced to spend the rest of his life in prison. Since then, the entire board of USA Gymnastics was forced to resign. The president of Michigan State University stepped down, and the university agreed to a half a billion dollar settlement with Dr. Nassar's accusers. In Washington, a new law was passed, requiring amateur sports organizations to report allegations of sexual abuse to law enforcement.

At the ESPY Awards this summer, Raisman and the other women who spoke out about Larry Nassar received a standing ovation as they were recognized for their courage.

Aly Raisman at ESPY Awards: to all the survivors out there, don't let anyone rewrite your story. Your truth does matter. You matter. And you are not alone.

Produced by Andy Court. Associate producers, Evie Salomon and Deborah Rubin.

This story originally aired on November 12, 2017.