Former sports doctor Larry Nassar provided the FBI with "instructional videos" in which the doctor worked on the private areas of girls, according to a report by the New York Times.
The Times published a report laying out a timeline of the FBI's investigation into Nassar, which raised serious questions as to how the biggest sexual abuse scandal in sports history unfolded.
Nassar first fell under FBI scrutiny in July 2015, after abuse allegations were reported to the agency by three gymnasts.
But the FBI took more than 12 months, from the time of the complaints, to interview and investigate victims claims.
During the 12 months, the FBI were provided with instructional videos produced by Nassar, in which he attempted to explain and demonstrate his procedures, including one he called "intravaginal adjustment".
Nassar demonstrated his "techniques" as he spoke clinically about pulled hamstrings, buttocks and trigger points.
The Times reported the videos showed Nassar kneading the legs of girls before moving his hands under a towel with ungloved hands between the girl's legs.
"It's not a fun place to dig," Nassar said to the camera.
"Do the hand-shaky thing," he added later as he shook his hand vigorously deep between a girl's legs.
When questioned, the FBI said the allegations "transcended jurisdictions" and that "there was vigorous debate as to whether Nassar was actually practicing a legitimate medical procedure".
"There is a duty to warn those who might be harmed in the future ... But everyone is still trying to ascertain whether a crime has been committed," said FBI agent W. Jay Abbott, who was in charge of the Indianapolis bureau.
He said "everybody had rights", including Nassar.
Despite the disturbing evidence, victims claimed the FBI didn't make contact for almost 14 months of the allegations.
Gina Nichols, mother of gymnast Maggie Nichols, said she was not contacted by the FBI for nearly a year after the information she provided sparked the federal inquiry.
"I never got a phone call from the police or the FBI ... Not one person. Not one. Not one. Not one," Nichols said.
At least 40 girls have reported they were sexually abused by Nassar during the time period of the FBI investigation.
Nassar was sentenced to 40 to 175 years in prison after pleading guilty to first-degree criminal sexual conduct of more than 260 young women.
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