But as the inquiry moved with little evident urgency, a cost was being paid. The New York Times has identified at least 27 girls and women who say that Dr. Nassar molested them between July 2015, when he first fell under F.B.I. scrutiny, and September 2016, when he was exposed by an Indianapolis Star investigation. Some are among the youngest of the now-convicted predator’s many accusers — 265, and counting.

The three alleged victims then at the center of the F.B.I.’s inquiry were world-class athletes; two were Olympic gold medalists. Nearly a year passed before agents interviewed two of the young women.

The silence at times drove the victims and their families to distraction, including Gina Nichols, the mother of the gymnast initially known as “Athlete A”: Maggie Nichols, who was not contacted by the F.B.I. for nearly 11 months after the information she provided sparked the federal inquiry.