The 63-year-old was facing charges that included human trafficking, criminal sexual conduct and lying to a peace officer, a release from Nessel's office read. Geddert had been expected to turn himself in and be arraigned on Thursday afternoon.
"My office has been notified that the body of John Geddert was found late this afternoon after taking his own life. This is a tragic end to a tragic story for everyone involved," Nessel said in a statement.
CNN has reached out to her office for further comment.
An attorney for Geddert, 63, did not immediately reply to a request for comment.
"It is alleged that John Geddert used force, fraud, and coercion against the young athletes that came to him for gymnastics training, for financial benefit to him," Nessel said. "The victims suffer from disordered eating including bulimia and anorexia, suicide attempts and self-harm, excessive physical conditioning, repeatedly being forced to perform even when injured, extreme emotional abuse and physical abuse including sexual assault."
CNN obtained a copy of the complaint, which says the alleged offenses stretch from 2008 to 2016.
A press release from Nessel's office said Geddert engaged in verbal, physical and sexual abuse against multiple young women.
Geddert was facing 14 counts of human trafficking-forced labor resulting in injury, six counts of human trafficking of a minor for forced labor, and one count each of continuing criminal enterprise, first-degree criminal sexual conduct, second-degree criminal sexual conduct and lying to a peace officer during a violent crime investigation.
The count of lying to a police officer stemmed from her office's investigation into Nassar, Assistant Attorney General Danielle Hagaman-Clark said. However, Hagaman-Clark said Geddert was being prosecuted for his alleged behavior, not that of Nassar.
"What needs to be reported is that these charges against Mr. Geddert are for his actions and his actions alone. They have very little to do with Mr. Nassar," she said. "Counts 1-20 that relate to human trafficking are for his behavior in the gym related to his coaching."
Geddert sold ownership of the club in 2018.
"The bravery of Geddert's many victims will stand for all time in stark contrast to his cowardice," Klein's statement read. "As a survivor and a mother of two young girls, my only comfort is in the knowledge that I can rest my head on the pillow every night knowing that John Geddert will never terrorize and abuse another child."
USA Gymnastics also released a statement on Geddert's death.
"This morning, we had hoped that news of the criminal charges being brought against John Geddert would lead to justice through the legal process. With the news of his death by suicide, we share the feelings of shock, and our thoughts are with the gymnastics community as they grapple with the complex emotions of today's events," the statement read.
At the time, Cook's office wouldn't comment on the type of complaints, type of investigation or how many people had come forward.
A year later, Michigan's attorney general said her office had taken over the investigation of Geddert.
CNN's Majlie de Puy Kamp, Laura Ly and Linh Tran contributed to this report.