(CNN) - Star gymnast Aly Raisman on Wednesday accused USA Gymnastics of being "100% responsible" for former team doctor Larry Nassar's years of sexual abuse, saying in a series of tweets that the organization enabled him and manipulated athletes.
"STOP VICTIM SHAMING," the two-time Olympian wrote in a tweet, tagging USA Gymnastics. "Your statements are hurtful. If you did not believe that I & others were abused than why pressure & manipulate us?
"WE WERE MOLESTED BY A MONSTER U ENABLED 2 THRIVE FOR DECADES. You are 100% responsible. It was mandatory to get 'treatment' by Nassar."
Raisman's tweets come a day after Maggie Nichols, a former USA Gymnastics star, said she also was a victim of Nassar's abuse. Her attorney said the organization and others failed the victims and should be held accountable.
USA Gymnastics on Tuesday said it hired an independent investigator to look into Nichols' claim at the time and then reported it to the FBI. The organization said it never tried to hide Nassar's misconduct. "USA Gymnastics kept the matter confidential because of the FBI's directive not to interfere with the investigation."
CNN reached out to the organization for a comment on Raisman's claims.
Raisman, who has previously called out USA Gymnastics regarding Nassar's abuse, asked for more investigation into what is described as the largest case of sexual abuse in sports history.
"We must investigate to figure out the many flaws in @USAGym system. We must understand how this happened to make sure it never occurs again. This problem is bigger than Penny and Nassar. The system has to change so that athletes are safe. Enablers need to be held accountable," Raisman wrote.
Steve Penny resigned as USA Gymnastics president last March.
Nassar, the disgraced former doctor for Michigan State University and USA Gymnastics, admitted in court to using his trusted medical position to sexually assault underage girls.
As the national governing body of gymnastics in America, USA Gymnastics is responsible for training and selecting the US national gymnastics team. Nassar was the team doctor for USA Gymnastics through four Olympic Games. He also worked at Michigan State from 1997 to 2016 as an associate professor and the gymnastics and women's crew team physician.
In December, Nassar was sentenced to 60 years in prison after pleading guilty to child pornography charges. He is expected to be sentenced to more prison time next week after pleading guilty to first-degree criminal sexual conduct charges.
More than 140 women and girls have accused him of sexual misconduct, including Olympic gold-medalists Raisman, McKayla Maroney and Gabby Douglas.
Who is to blame?
Nichols, now 20, said she and a coach reported Nassar's abuse to USA Gymnastics officials in 2015, a few years after he began abusing her when she came to him for a back injury.
"USA Gymnastics and the USOC did not provide a safe environment for me and my teammates to train. We were subjected to Dr. Nassar at every National Team Camp, which occurred monthly at the Karolyi Ranch," Nichols said, referring to the training center operated by Bela and Martha Karolyi.
"His job was to care for our health and treat our injuries. Instead, he violated our innocence," she said.
Nichols' attorney, John Manly, accused USA Gymnastics of a "brazen attempt to cover up the largest child sex abuse scandal in sports."
In its response, USA Gymnastics cited Nichols' "bravery" and said it reported Nassar to the FBI in July 2015 and to a different FBI office in April 2016. The group said information from Nichols and another athlete "was important, but did not provide reasonable suspicion that sexual abuse had occurred." Following an interview with a third athlete, information from all three young women was given to the FBI, it said.
"Contrary to reported accusations, USA Gymnastics never attempted to hide Nassar's misconduct," the group said. "The suggestion by plaintiff's counsel ... that USA Gymnastics tried to silence athletes or keep the investigation secret to avoid headlines before the Rio Olympics and to protect Los Angeles' Olympic bid is entirely baseless."
USA Gymnastics adopted a series of reforms in June that it said will better prevent and respond to future cases of abuse.
An attorney for Nassar, Matthew Newburg, told CNN on Tuesday that he had no comment on Nichols' allegations.
In a statement, Manly said he represents 107 women and girls in civil lawsuits targeting USA Gymnastics and Michigan State University. Manly has accused both organizations of failing to properly investigate allegations of abuse or take appropriate actions.