Minnesota's program for keeping sex offenders confined after they complete their prison sentences is constitutional, a federal appeals court ruled Tuesday, reversing a lower-court judge who said it violates offenders' rights because hardly anyone is ever released.

A three-judge panel of the 8th U.S. Circuit Court of Appeals sided with the state, which argued that the program is both constitutional and necessary to protect citizens from dangerous sexual predators who would otherwise go free. The appeals court sent the case back to the lower court for further proceedings.

Only six offenders are currently free on provisional releases from the Minnesota Sex Offender Program, even though it's more than 20 years old. That led U.S. District Judge Donovan Frank in 2015 to declare the program unconstitutional and order changes to make it easier for people to get on a pathway for release.

The Minnesota case has been closely watched by lawyers, government officials and activists in the 20 states with similar programs. While civilly committed offenders in California, Wisconsin, New Jersey and other states are allowed to re-enter society after completing treatment, Minnesota has the highest per capita lockup rate, and its courts didn't order the unconditional release of anyone from its program until August.

Minnesota's offenders are confined by court order for treatment at secure facilities in Moose Lake and St. Peter that are ringed by razor wire, though there's a section outside the wire at St. Peter for people who've progressed to the later stages of treatment and been given some limited freedoms. They're officially considered patients or residents, not prisoners. But the lawsuit filed on behalf of more than 700 offenders argued that the program amounts to a life sentence.