The guilty pleas continue to rack up for Matthew Muller, the man convicted in the infamous 2015 kidnapping and sexual assault of Denise Huskins which was the subject of Netflix's American Nightmare.

Muller, 48, pleaded guilty Wednesday to a 1993 kidnapping and sexual assault that he admitted to committing in Folsom, in Sacramento County, when he was 16 years old. As KPIX reports, Muller approached a campsite near Folsom Lake and ordered a man and a woman out of their tent at gunpoint. He tied up the male victim, and then took the female victim to a different location and assaulted her, prosecutors said.

The case, which was apparently Muller's first thrill as a budding kidnapper and rapist, was one of several that Muller admitted to in correspondence from jail with the chief of police in Seaside, California, Nick Borges. Huskins collaborated with Borges to begin a correspondence with Muller last year, believing him to be a serial predator who had likely committed multiple similar crimes that he might now admit to.

Muller, who says he's become a born-again Christian in prison, told Borges that he wanted to come clean about his earlier crimes, prior to the kidnapping and assault of Huskins, out of feeling a "common goal" with law enforcement for "strengthening laws for future potential victims."

Muller confessed to two home invasions on the San Francisco Peninsula in 2009 which predated the Huskins case by six years. In both cases, he snuck into the homes of women residing alone, tied them up, and either threatened to or succeeded in assaulting them.

Muller has been serving a 40-year federal prison sentence in Tucson, Arizona for charges relating to the Huskins case, and a concurrent 31-year sentence in a state conviction in the same case. He was then extradited to Santa Clara County in December and sentenced to two more life terms for the Peninsula cases.

He has yet to face prosecution in the Contra Costa County case.

Previously: Matthew Muller, Kidnapper and Rapist In the 'American Nightmare' Story, Sentenced to Two More Life Terms