(Reuters) - Eighteen female athletes on Tuesday sued a sports doctor to Olympian gymnasts, saying he sexually assaulted them under the guise of medical examinations over two decades when most of them were minors.
Larry Nassar, once a team doctor for Olympians at USA Gymnastics and a former employee at Michigan State University, has been in federal custody since December on child pornography charges. He also faces separate charges in Michigan for sexual abuse of a minor, who was neither an athlete nor a patient when police say she was abused from ages 6 to 12.
Nassar is also under investigation by Michigan authorities who say they have received more than 50 complaints of sexual abuse by the doctor.
The lawsuit says Michigan State University failed to act on complaints about Nassar from two of the plaintiffs in 1999 and 2000.
It also says USA Gymnastics, the U.S. governing body for the sport, failed to alert Michigan State when it learned about Nassar's alleged misconduct in 2015.
An attorney for Nassar did not immediately respond to a request for comment. His attorneys have denied any misconduct in the past, and he has pleaded not guilty to the state and federal criminal charges.
The lawsuit accuses Nassar of treating hip, leg, back and ankle injuries with what he called "inter-vaginal adjustments" that involved digital penetration without the consent of the athletes or their parents.
The athletes, aged 9 to 29 at the time of the allegations, participated in gymnastics, softball, basketball, soccer, figure skating and swimming, the lawsuit said.
Michigan State and USA Gymnastics both said in separate statements they could not comment on the lawsuit but that they were cooperating with criminal investigators.
Most of the accusers were unnamed but plaintiff Rachael Denhollander has identified herself publicly, saying she was repeatedly sexually assaulted by Nassar "under the guise of medical examinations" when she was 15 years old.
USA Gymnastics said it immediately began cooperating with the Federal Bureau of Investigation in 2015, when it severed ties with Nassar upon learning of athlete concerns.
Michigan State said it started its own investigation in 2014 and that no other accusers came forward until 2016.
"Our hearts go out to those directly affected," the university said in a statement. "The criminal investigation into Larry Nassar is a top priority for MSU Police."
Nassar faces another civil lawsuit in California from an unnamed Olympian accusing him of sexual misconduct.
Reporting by Daniel Trotta in New York; Editing by Lisa Shumaker