Story highlights Larry Nassar is already facing state and federal charges for sex assault, child pornography

Michigan State University has launched an investigation

(CNN) Eighteen women and girls have filed a federal lawsuit against disgraced former USA Gymnastics and Michigan State University doctor Larry Nassar, alleging that he sexually assaulted them repeatedly for years during physical examinations.

The plaintiffs alleged a pattern of sexual abuse, disguised as treatment, that began in the late 1990s and continued into 2016. Most of the girls were teenagers at the time of the alleged assaults, although several were as young as 9.

Nassar, 53, is already facing state and federal criminal charges, including sexual assault with a person under 13 and possession of child pornography.

CNN reached out to Matt Newburg, Nassar's attorney in the criminal cases, who said he does not represent him in the civil case. It could not be immediately determined whether Nassar has retained an attorney in the civil suit.

Seventeen of the plaintiffs in the civil suit, filed Tuesday, have chosen to remain anonymous. The other, Rachael Denhollander, says she was assaulted by him repeatedly at Michigan State in 2000, when she was 15 and seeking treatment for wrist and back pain.

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