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A doctor accused of molesting Olympic gymnasts and scores of other patients should spend the rest of his life behind bars for a "abominable" child-pornography collection that fed his urges, federal prosecutors said Thursday.

In a pre-sentencing memo, the U.S. Attorney's Office in Michigan asked a judge to sentence Larry Nassar to 60 years in prison, saying he's a danger to society.

Nassar, 54, was already facing a minimum of 25 years in prison after pleading guilty in two state courts to sexually assaulting 10 girls.

Related: Ex-gymnastics doctor Larry Nassar pleads guilty to 3 more criminal sex charges

Although Nassar pleaded guilty only to pornography charges in the federal case, prosecutors argued his long history as a sexual predator should be a factor in the sentence he will receive next week.

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They quoted one of his victims as saying, "I became his real-life child pornography subject, his plaything and his experiment."

The prosecutors have asked for six of Nassar's victims or their relatives to speak at the federal sentencing, but have not identified them.

Nassar's accusers include some of the biggest names in the gymnastics world. In recent weeks, gold medalists McKayla Maroney, Ali Raisman and Gabby Douglas have also said they were molested by him under the guise of invasive exams and treatments.

Related: Olympics star Gabby Douglas says team doctor Larry Nassar abused her

The longtime team doctor for USA Gymnastics "has led a double-life," prosecutors wrote in a memo.

"On the surface, he was a respected, world-renowned expert for elite athletes. He was a medical doctor, a husband, and a father. But underneath this veneer lurked a predator," they added.

Larry Nassar appears in Ingham County Circuit Court earlier this month. Jeff Kowalsky / AFP - Getty Images file

"For 20 years, the defendant used his position and authority to sexually exploit children. He took advantage of almost every opportunity to do so. His patients, his friends’ children, and the Internet were there for the plucking.

"To be clear, this defendant amassed an enormous collection of abominable images of children being sexually abused, raped and degraded. He acted on his vile urges and sexually abused scores of girls and young women," the government's memo said.

Nassar initially pleaded not guilty to child molestation charges and insisted his "procedures" on girls and young women were medically sound. He said in court last week that he was changing his plea to help the community heal.