ST. LOUIS (AP) — A trial regarding the constitutionality of a Missouri Department of Mental Health program for people convicted of sex crimes has begun.

The St. Louis Post-Dispatch reports the trial began Tuesday. A class action lawsuit alleges the Sex Offender Rehabilitation and Treatment Services program is a prison in disguise.

Missouri law identifies sex offenders nearing the end of their prison terms, and allows a process for civil courts to decide if inmates need to be kept from the community, but in treatment rather than in prison. There is no time limit for confinement, but state law says treatment should continue until the inmate’s risk to society falls.

The plaintiffs’ attorney says nobody has been released for successfully completing the program since it was created in 1999.

The Missouri attorney general’s office, which is defending the state, says the legality of the program is sound.