McKayla Maroney, a two-time Olympic medalist, has alleged that she was abused as early as age 13 by former Michigan State and USA Gymnastics doctor Larry Nassar, who already faces 22 counts of first-degree criminal sexual conduct and could receive a sentence of life in prison.

Maroney, now 21, helped the "Fierce Five" U.S. women's gymnastics team to the gold medal at the 2012 Summer Games in London. She spoke out early Wednesday as part of the "#MeToo" movement on social media, saying, "Silence has given the wrong people power for too long, and it's time to take our power back."

Nassar was involved with USA Gymnastics for nearly three decades as a trainer and national medical coordinator, a role that led him to treat the country's elite gymnasts at four separate Olympic Games.

Maroney said the abuse began when she was 13 and attending a U.S. National team training camp at the Karoyli Ranch in the Sam Houston Forest north of Houston, Texas. She did not detail Nassar's specific actions.

"Dr. Nassar told me that I was receiving 'medically necessary treatment that he had been performing on patients for over 30 years,'" Maroney wrote. "It started when I was 13 years old, at one of my first National Team training camps, in Texas, and it didn't end until I left the sport. It seemed whenever and wherever this man could find the chance, I was 'treated.'

"It happened in London before my team and I won the gold medal, and it happened before I won my silver [in vault in London]."

Maroney also wrote that when she was 15, Nassar gave her a sleeping pill during the team's all-day flight to Tokyo for the 2011 world championships and that she woke up alone with him in his hotel room.

"For me, the scariest night of my life happened when I was 15 years old," Maroney wrote in her Twitter post. "... He'd given me a sleeping pill for the flight, and the next thing I know, I was all alone with him in his hotel room getting a 'treatment.' I thought I was going to die that night."

Attorneys for Nassar had no comment when reached by The Associated Press.

USA Gymnastics praised Maroney's strength in a statement on Wednesday, adding it is "outraged and disgusted" by Nassar's alleged conduct.

"We are strengthening and enhancing our policies and procedures regarding abuse, as well as expanding our educational efforts to increase awareness of signs to watch for and reporting suspicions of abuse, including the obligation to immediately report," USA Gymnastics wrote. "USA Gymnastics, its members and community are committed to working together to keep our athletes as safe as possible."

Former "Fierce Five" teammates Aly Raisman, Jordyn Wieber and Kyla Ross also offered their support for Maroney on Twitter.

100% support you. SO proud of you and your strength. Love you like a sister!! https://t.co/1VHb79a7lx — Alexandra Raisman (@Aly_Raisman) October 18, 2017

So sad and heartbreaking that this happened to you Mack. I'm here for you and I support you https://t.co/LXA1Yf04FC — Jordyn Wieber (@jordyn_wieber) October 18, 2017

Heartbreaking that things like this can happen in this world😭 My heart goes out to you mac & all those affected by such tragic events🙏🏽❤️ https://t.co/9KsKtC2a2y — Kyla Ross (@kyla_ross96) October 18, 2017

Attorneys representing 125 women who are suing Nassar agreed in August to take the cases to mediation, a step that could result in a settlement.

The plaintiffs are also suing USA Gymnastics, Michigan State and other defendants. Attorneys have said that more women are expected to join the lawsuits.

It is not known whether Maroney, who lives in California and officially retired in 2015, is part of those lawsuits.

Nassar has already pleaded guilty to federal child pornography charges and is in jail, awaiting a Nov. 27 sentencing in that case.

He also faces charges in state court in Michigan, largely related to allegations that he digitally penetrated women during medical exams for his own sexual gratification. If convicted on any one of the 22 counts of first-degree criminal sexual conduct, Nassar could be sentenced to life in prison.

In the past, Nassar and his attorneys have defended the intra-vaginal and intra-rectal procedures as accepted medical practice.

Maroney called for change, urging other victims of abuse to speak out and demanding organizations "be held accountable for their inappropriate actions and behavior."

"This is happening everywhere," Maroney wrote. "Wherever there is a position of power, there is the potential for abuse. I had a dream to go to the Olympics, and the things I had to endure to get there, were unnecessary and disgusting."

Maroney is the highest profile gymnast yet to come forward publicly claiming abuse by Nassar. Jamie Dantzscher, a bronze medalist on the 2000 U.S. Olympic team, was part of the initial wave of lawsuits filed against Nassar in 2016.

Raisman, who won six medals while serving as the captain of the U.S. women's team in both 2012 and 2016, called for sweeping change at USA Gymnastics in August.

USA Gymnastics launched an independent review of its policies in the wake of the allegations against Nassar in the summer of 2016 following reporting by the Indianapolis Star that highlighted chronic mishandling of abuse allegations against coaches and staff at some of its more than 3,500 clubs across the country.

In June, the federation immediately adopted 70 recommendations proffered by Deborah Daniels, a former federal prosecutor who oversaw the review. The new guidelines require member gyms to go to authorities immediately, with Daniels suggesting USA Gymnastics consider withholding membership from clubs that decline to do so. The organization also named Toby Stark, a child welfare advocate, as its director of SafeSport. Part of Stark's mandate is educating members on rules, educational programs, reporting and adjudication services.

The organization also fired president Steve Penny in March. A replacement has not been named.

ESPN's John Barr and The Associated Press contributed to this report.