The state of Minnesota is moving to block the release of Thomas Duvall, a convicted serial rapist who was at the center of a political storm two years ago over how to fix the state's troubled sex offender program.

State Human Services Commissioner Emily Johnson Piper has formally appealed a decision last month by a state review board to grant provisional discharge to Duvall, 60, who was convicted on three separate occasions of sexually assaulting teenage girls — each time shortly after he was released from prison.

In 2014, Duvall's approval for conditional discharge set off a political furor, ensnaring two prominent public prosecutors in a feud, and prompting Gov. Mark Dayton to temporarily suspend all releases from the Minnesota Sex Offender Program (MSOP).

Piper's appeal sets the stage for another long and bruising legal fight over Duvall's future and comes as the state faces heightened pressure to show that MSOP operates a viable treatment program and is not a "de facto life sentence," as critics allege.

"There will be a battle, for sure," warned Warren Maas, president of the Minnesota chapter of the Association for the Treatment of Sexual Abusers. "It's just too hot a political case to pass quietly."

The case of Duvall, who was civilly committed in 1991, is emblematic of Minnesota's unusually complicated and often-politicized process for treating and releasing offenders who have been civilly committed for treatment after serving their prison terms. Last July, a federal judge in St. Paul declared MSOP unconstitutional for detaining offenders indefinitely while depriving them of access to the courts and regular risk evaluations, among other safeguards.

Like many of the 725 offenders confined at MSOP, Duvall has spent years mired in an administrative and legal web. In 2010, he filed a petition for discharge but withdrew it two years later after an expert identified unresolved issues with his treatment. In 2013, Duvall petitioned again and was granted provisional discharge by a three-member panel. The Department of Human Services did not object to his long and detailed plan for release to a halfway house, which included 27 conditions.

But in the fall of 2014, state and county officials abruptly reversed course and opposed his release, after Attorney General Lori Swanson and others raised alarms about the number and brutal nature of Duvall's sexual assaults. In one particularly horrific incident, on Dec. 26, 1987, Duvall bound a 17-year-old girl with an electric cord and repeatedly raped her while hitting her with the handle of a hammer.

His case was also complicated by reports that MSOP staff had shredded personal journals kept by Duvall. Swanson's office alleged a coverup, claiming Duvall's journals contained evidence that he still fantasized about sexual violence.

As Duvall's petition for release unraveled in 2014, Dayton, who was seeking re-election, lashed out at the "political circus" surrounding the case and ordered state officials to suspend releases until the Legislature reviewed MSOP.

Even now, nearly two years later, much of Duvall's case remains shrouded in mystery. The findings of the three-person special review board that last month approved his petition are filed under seal with the state courts. Piper declined to offer specifics on the state's appeal, citing privacy concerns. The Hennepin County attorney's office also is opposing his release.

Eric Janus, a professor at Mitchell Hamline School of Law in St. Paul and author of a book on sexual predator laws, said Duvall's case stands as "an important test case of the legitimacy" of Minnesota's civil commitment system and whether decisions by treatment specialists and panels can survive political pressure.

As the federal courts have intensified their scrutiny of the program, Minnesota has quietly moved more offenders toward release. Over the past 16 months, six offenders have been granted conditional release, compared with only two in the program's prior 20 years. More than 60 have been approved for transfer to dormlike settings on MSOP's St. Peter campus, a final step before release into the community.

Duvall's case now goes before a three-judge, state Supreme Court Appeals panel. A hearing before the panel is likely by January 2017, though one has not been scheduled.

Chris Serres • 612-673-4308

Twitter: @chrisserres