A longtime doctor for the American gymnastics team was charged with sexual assault on Wednesday in Michigan.

The doctor, Lawrence G. Nassar, faces 22 charges of first-degree criminal sexual conduct involving at least seven victims. Prosecutors said that they expected more charges and that they were hearing from more victims.

“This guy is a monster,” Bill Schuette, the attorney general of Michigan, said at a news conference on Wednesday. “You read those affidavits and it shocks you to the very core.”

The affidavits describe the accounts of victims as young as 10 who say Dr. Nassar digitally penetrated them and molested them under the guise of performing medical treatment. Many were repeatedly assaulted over the course of years.