A Gillette man faces 35 years of imprisonment after pleading guilty to two counts of production of child pornography in federal court today.

Charles Mathisen entered the pleas before Chief U.S. District Court Judge Nancy Freudenthal in Cheyenne.

In exchange for the pleas, the government will ask that two counts of possession of child pornography and five other counts of production of child pornography will be dismissed at Mathisen's sentencing, which Freudenthal set for Dec. 12.

Conviction of production of child pornography is punishable by 35 years to life imprisonment.

Mathisen is in his early 60s, which means he probably will spend the rest of his life behind bars. He also has to pay a $5,000 fine to the Victims of Sex Trafficking Act of 2015. If he is released before he dies, he will be on probation for five years to life.

He remains in custody pending his sentencing.

The case started in May 2016 when the Gillette Police Department began the investigation of a sexual assault of a prepubescent girl, according to the criminal complaint filed by a Gillette police officer.

Police executed an arrest warrant for Mathisen on a charge of first-degree sexual abuse of a minor.

Police also executed a search warrant on his residence. Police seized a computer, digital camera and other electronic devices.

The officer analyzed images and found Mathisen had photographed and made video files of the girl while he was assaulting her, according to the affidavit.

Mathisen also admitted to detectives he used a camera to photograph and video the girl.

On Jan. 27, a doctor examined the victim and positively identified her as the girl in the images.

Mathisen initially was charged with one count of possession of child pornography in a criminal complaint filed June 8.

The grand jury indicted him on July 20 and charged him with two counts of possession of child pornography and seven counts of production of child pornography.

The indictment says Mathisen enticed a girl under the age of nine, and a girl under the age of seven to engage in sexually explicit conduct so he could video-record it.