Prison officials incorrectly allowed 1,500 inmates to be placed on unsupervised parole last year, including 450 who should have been classified as having a "high risk for violence," a new audit of the state's parole programs has found.

The report, which comes at a critical period for state corrections officials under court mandate to reduce prison populations by 33,000 inmates, came from the state inspector general's office.

It found that a 2009 program for the California Department of Corrections and Rehabilitation to place inmates on non-revocable parole incorrectly classified many of them initially, allowing hundreds to be returned to their communities with no supervision at all.

Corrections officials disputed the findings of the 34-page report, which does not indicate whether any of the parolees who were improperly classified went on to commit new crimes.

The report concedes that errors in classifying parolees have since been corrected. It also notes that the non-revocable parole program will be phased out as a result of Gov. Jerry Brown's plans to turn over supervision of tens of thousands of nonviolent, nonserious inmates to counties if the funding is approved.

State Sen. Ted Lieu, D-Torrance, who asked for the study, called it a "scathing report" and said the findings show more than 1,000 potentially violent inmates could have been released without supervision last year.

"I had anecdotal evidence that CDCR was releasing violent, high-risk offenders and putting them inappropriately on unsupervised parole," Lieu said. "But I did not realize the scale of what they were doing."

Lee Seale, deputy chief of staff to Corrections Secretary Matthew Cate, responded to the report with a three-page letter refuting many of the inspector general's findings. He wrote that it was "unfortunate" the report focused on a program that is expected to end and on "alleged 'errors' " that have since been corrected.

But the report raises questions about how the parole program initially was handled by corrections officials.

The non-revocable parole program was hatched with the passage of legislation in 2009 designed to take low-level offenders out of the prison system. Thousands of these offenders go in and out of prison on a regular basis for minor parole violations, causing overcrowded conditions and placing a strain on the prison system.

The logic was that offenders whose record shows they pose little threat to society could be kept out of prison by placing them on parole with no supervision and no threat of going back into custody on a parole violation.

To be returned to prison, they would have to commit a new felony.

By last December, the California Department of Corrections and Rehabilitation had 14,859 offenders on non-revocable parole, the report found.

Corrections officials installed a computer program to assess an inmate's likelihood of reoffending and in March 2010 reported that refinements to it had determined more than 600 inmates had been placed on non-revocable parole who should not have been.

Eventually, about 400 of those were reclassified and required to be on supervised parole, meaning they had to report to parole agents regularly and could not violate conditions of their release without the threat of being returned to prison.

The corrections department estimates that nearly half of parolees who are supervised end up being returned to prison for various violations.

"It is therefore probable that some of the discharged parolees inappropriately placed on non-revocable parole would have violated their parole conditions and returned to prison, had they been on supervised parole," the report concludes.

The report comes on the heels of Monday's U.S. Supreme Court decision requiring California's prison population of more than 143,000 to be reduced by 33,000 inmates in the next two years.

Corrections officials say most of the reductions required by the court can be handled through the realignment plan of shifting low-level offenders into county jails.

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