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A lawsuit challenging the constitutionality of the Minnesota Sex Offender Program can move forward as a class-action effort involving more than 600 people who have been indefinitely committed to the program, a federal judge ruled Tuesday.

U.S. District Judge Donovan Frank ruled that the lawsuit meets the legal requirements for a class action. He concluded that addressing each case individually would be an "enormous drain" on legal resources.

The sex offenders who've been committed to the program have all finished their prison sentences, but prosecutors and the courts have deemed them too dangerous to be released without treatment. They're officially considered patients, not prisoners, but they're not free to leave. Until one patient earned a provisional discharge into a halfway house earlier this year, none had been successfully released since the program began in 1994.

Frank wrote that all the offenders in the program face an identical process for treatment and possible release, raise similar allegations about a lack of realistic opportunities for earning their freedom, and have sufficiently similar legal interests for their cases to go forward together.

The lawsuit levels several allegations against the state, including failure to provide adequate treatment. It also challenges the constitutionality of the state's civil commitment statute. While the courts have upheld the constitutionality of the program before, it's been dogged by persistent questions about costs and the lack of anyone being released.

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"This lawsuit is saying in general terms that you can't commit people for medical treatment and then close the door forever," Minneapolis attorney Daniel Gustafson, who is representing the plaintiffs, told the Star Tribune. "Whether there's a settlement that can be reached in this case or not remains to be seen. But certainly if there is a settlement, it's going to have to address this notion that being committed under the Minnesota sex offender program is a life sentence."

The Minnesota Sex Offender Program was introduced in 1994, but its population dramatically increased after the 2003 kidnapping, rape and murder of 22-year-old Dru Sjodin, according to the newspaper. Sjodin, a student at the University of North Dakota, was abducted from the parking lot of the Columbia Mall in Grand Forks, N.D., in November 2003. Her body was found in April 2004 just west of Crookston, Minn.

It was later revealed that Sjodin's killer, Alfonso Rodriguez, Jr., had served only prison time for the stabbing and attempted kidnapping of another woman prior to abducting Sjodin.

Rodriguez has been sentenced to death for killing Sjodin.

The Associated Press contributed to this report.