A convicted serial rapist should be allowed to be released into the community under supervision, the Minnesota state Court of Appeals ruled Monday, saying the state did not prove by clear and convincing evidence that Thomas Duvall should remain in treatment.

Department of Human Services Commissioner Emily Piper said Monday that she will appeal the provisional discharge of Duvall, in a case that once set off a political firestorm as lawmakers were considering changes to the state's treatment program for sex offenders.

"I have grave concerns about this decision," Piper said in a statement. "Three experts have previously testified that Thomas Duvall is not ready for life in the community and that he presents far too great a risk to public safety. I share that view and will exhaust every possible avenue of appeal."

Duvall, 62, has spent the last 30 years locked up for the violent rapes of teenage girls in the 1970s and 1980s. In 1987, he bound a Brooklyn Park girl with an electrical cord and raped her repeatedly over several hours while hitting her with a hammer. He was civilly committed as a psychopathic personality in 1991 and sent to the Minnesota Sex Offender Program.

Duvall has been in treatment since 2001 and was diagnosed as a sexual sadist. He has been in the final stages of the program since 2010, living outside the security perimeter at the facility in St. Peter, going on regular supervised community outings, volunteering at a thrift store, attending community support groups and preparing for transition into the community.