Thomas Duvall was convicted of a series of brutal rapes of teenage girls in the 1970s and has admitted to more than 60 victims. (The Minnesota Star Tribune)

State and county officials are challenging the proposed release of serial rapist Thomas R. Duvall, one of the most violent sex offenders in state history, from the Minnesota Sex Offender Program (MSOP).

In court filings this week, state Human Services Commissioner Emily Piper and Hennepin County prosecutors argued that Duvall's conditional release, recommended by MSOP staff and approved by a state judicial panel, was opposed by all the expert evaluators "not employed by the MSOP" who offered testimony in the case. That includes a court-appointed psychologist, Dr. James Alsdurf, who described Duvall as a "sexual sadist" still fixated on violent sexual thoughts in court testimony last spring.

The petitions could delay or even derail plans to release Duvall, 62, who has been confined at the MSOP since 1991 for a series of brutal rapes of teenage girls in the 1970s and 1980s and who has admitted to assaulting more than 60 victims. In a particularly horrific assault in 1987, Duvall bound a 17-year-old girl from Brooklyn Park with an electric cord, cut off her clothing with a knife, and repeatedly raped her over a three-hour period while hitting her with a hammer. He also assaulted a 15-year-old girl on her way to school; and later that day, he sexually assaulted two other girls ages 14 and 15, using a shotgun to threaten them, according to court documents.

Duvall was later committed by a judge to the MSOP after serving prison time for his crimes.

"I appealed the court's decision to release Thomas Duvall because three expert evaluators who testified in this case believe he continues to pose a risk to public safety," Piper said in a written statement.

On Jan. 8, a three-judge state appeals panel approved Duvall's petition for release, saying he was "capable of making an acceptable adjustment to open society" after nearly 30 years in state custody.

The panel relied heavily on members of Duvall's treatment team at the MSOP, who described him as a model detainee in testimony last April. Staff at the MSOP noted that Duvall had gone on more than 100 community outings and had worked at a thrift store for eight years without any incidents of inappropriate behavior; he also had a long history of continuous participation in sex offender treatment.

"[Duvall] cannot change his past offense history, but he is committed to change in the present and future," the panel wrote in its 42-page ruling earlier this month.

The panel's decision alarmed some of Duvall's victims and their families across the state, who are concerned about his ability to control his deviant sexual urges. As part of his therapy at the MSOP, Duvall maintained personal journals, or "fantasy logs," describing his inner thoughts. In these logs, which span more than 500 pages, Duvall described fantasies involving teenage girls, female body parts and past victims.

Alsdurf, the psychologist appointed by the court, cited the journals as evidence that Duvall was still "obsessed with sex — most of it violent," and testified against his release last year.

A new hearing on Duvall's case has been scheduled for April 18 before a three-judge panel of the Minnesota Court of Appeals in St. Paul. The judges must issue a decision within 90 days of oral arguments.