Two people familiar with the matter told The Associated Press the attack happened on Sunday at United States Penitentiary Coleman. The people said he was in stable condition on Monday.

One of the people said Nassar had been stabbed in the back and in the chest.

The people were not authorised to publicly discuss details of the attack or the ongoing investigation and spoke to the AP on the condition of anonymity.

Nassar is serving decades in prison for convictions in state and federal courts. He admitted sexually assaulting athletes when he worked at Michigan State University and at Indianapolis-based USA Gymnastics, which trains Olympians. Separately, Nassar pleaded guilty to possessing child pornography.

During victim impact statements in 2018, several athletes testified that over the course of Nassar's more than two decades of sexual abuse they had told adults, including coaches and athletic trainers, what was happening but that it went unreported.

More than 100 women, including Olympic gold medalist Simone Biles, collectively sought more than USD 1 billion from the federal government for the FBI's failure to stop Nassar when agents became aware of allegations against him in 2015. He was arrested by Michigan State University police in 2016, more than a year later.

The Justice Department's inspector general said in July 2021 that the FBI made “fundamental” errors in investigating the sexual abuse allegations against Nassar and did not treat the case with the “utmost seriousness”.

More athletes said they were molested before the the FBI swung into action.

The inspector general's investigation was spurred by allegations that the FBI failed to promptly address complaints made in 2015 against Nassar. USA Gymnastics had conducted its own internal investigation, and the organisation's then-president, Stephen Penny, reported the allegations to the FBI's field office in Indianapolis. But it took months before the bureau opened a formal investigation.

At least 40 girls and women said they were molested over a 14-month period while the FBI was aware of other sexual abuse allegations involving Nassar. Officials at USA Gymnastics also contacted FBI officials in Los Angeles in May 2016 after eight months of inactivity from agents in Indianapolis.

The FBI acknowledged conduct that was “inexcusable and a discredit” to America's premier law enforcement agency.

Michigan State, which was accused of missing chances over many years to stop Nassar, agreed to pay $500 million to more than 300 women and girls who were assaulted by him. USA Gymnastics and the US Olympic and Paralympic Committee made a USD 380 million settlement.

In June 2022, the Michigan Supreme Court rejected a final appeal from Nassar. Attorneys for Nassar said he was treated unfairly in 2018 and deserved a new hearing, based on vengeful remarks by a judge who called him a “monster” who would “wither” in prison like the wicked witch in “The Wizard of Oz.”

“I just signed your death warrant,” Ingham County Judge Rosemarie Aquilina said of Nassar's 40-year sentence.

The state Supreme Court said that Nassar's appeal was a “close question” and that it had “concerns” over the judge's conduct. But the court also noted that Aquilina, despite her provocative comments, stuck to the sentencing agreement worked out by lawyers in the case.

“We decline to expend additional judicial resources and further subject the victims in this case to additional trauma where the questions at hand present nothing more than an academic exercise,” the court said in a two-page order.

More than 150 victims spoke or submitted statements during an extraordinary seven-day hearing in Aquilina's court more than four years ago.

“It's over. ... Almost six years after I filed the police report, it's finally over,” said Rachael Denhollander, the first woman to publicly accuse Nassar.