Larry Nassar molested 40 women and girls as a 14-month FBI investigation into the disgraced former Olympic team doctor inched along, the New York Times claimed on Saturday.

The FBI first became aware of claims made against Larry Nassar in July 2015 but the full extent of allegations made against him did not become public knowledge until September 2016 following an exposé by a Indianapolis newspaper.

During this time, Nassar, 54, continued to work with athletes and dancers at Michigan State University where it is believed he molested at least 40 more victims. USA Gymnastics cut ties with the doctor in 2015.

In January, Nassar was convicted of numerous sexual assaults, dating back decades, and sentenced to 40-to-175 years by a court in Michigan. So far, 250 victims, including high-profile Olympic medallists Simone Biles and McKayla Moroney have come forward say there were assaulted while the doctor administered medical treatment.

In November 2017, Nassar plead guilty to numerous charges of first degree sexual conduct with minors under the age of 16. Nassar was already serving a 60 years sentence for possession of over 37,000 images of child pornography . He will receive a further sentence on Monday.

According to W. Jay Abbbot, who led the FBI's Indianapolis branch during the investigation, parents and other victims were not warned.

"That's where things can get tricky", he said. "There is a duty to warn those who might be harmed in the future," said Abbott, who retired in January. "But everyone is still trying to ascertain whether a crime has been committed."

He said "everybody has rights here", including Nassar himself.