SACRAMENTO -- -- Sara Jane Olson's premature release from state prison last week was the result of a clerical error made three years ago, officials said Monday.

The mistake occurred in early 2005, when a worker updated the former Symbionese Liberation Army member's prison file and inadvertently cut two years off her term.

Olson was freed March 17, then rearrested Saturday after being detained at Los Angeles International Airport as she prepared to fly home to Minnesota. She will have to serve only one more year if she works and remains free of discipline while in prison.

Olson, formerly known as Kathleen Soliah, had been given a 16-year term for two crimes by the state parole board in 2002. She received 14 years for a 1975 plot to kill Los Angeles police officers by blowing up their patrol cars, and two years for second-degree murder in a Sacramento bank robbery during which a customer was killed by another SLA member.

The SLA was best known for kidnapping newspaper heiress Patricia Hearst. After the crimes, Olson, now 61, spent 24 years as a fugitive before being apprehended in 1999 in St. Paul, Minn.

After she was sent to prison, her lawyers challenged the sentence in the Los Angeles case. Based on a ruling by a state Superior Court judge, the parole board held a hearing in 2004 and ordered Olson's sentence in that case reduced from 14 to 13 years, said Alberto Roldan, chief deputy general counsel for the California Department of Corrections and Rehabilitation.

Counting the two years she received in the Sacramento case, Olson's sentence should have been 15 years, according to Roldan, who said he reviewed her file Saturday. But a correctional case records worker read the minutes of the parole board hearing and, in updating the file, reduced Olson's sentence too much.

The employee "inadvertently entered it in, reducing the overall term . . . to 13 years," Roldan said.

He said that error occurred Jan. 18, 2005, most likely at the Central California Women's Facility in Chowchilla, where Olson was incarcerated.

From that point on, the mistake was codified in Olson's case file. Last year, based on another court challenge by her lawyers, the state knocked another year off her sentence in the Los Angeles case.

With credit for good behavior and work she performed in prison, she was released last week after serving six years. Olson worked on a maintenance crew that swept and cleaned the main yard of the Chowchilla lockup, according to prison officials.

After news of her release became public, Sacramento County prosecutors expressed concern that Olson had not served enough time for the crime she had committed in their jurisdiction. State officials investigated and discovered the mistake.

Her earliest possible release now is March 17, 2009, officials said. At that point, she will have served half of her 14-year term.

She was taken to a prison in Chino on Saturday and has since been moved back to Chowchilla.

The back-and-forth over Olson's prison term resulted from a change in California law from indeterminate sentences -- which gave prison officials more flexibility in deciding when an inmate was ready for release -- to determinate, or fixed, sentences. Because Olson's crimes were committed before the law changed, the parole board had to convert her sentence from the old system to the new one.

The conversion was the focus of the appeals by her lawyers, which led to the repeated changes in her sentence and, ultimately, the error that set her free early.

Such calculations are so complicated that even those involved with Olson's cases were struggling to understand them Monday.

The Sacramento County district attorney, Jan Scully, issued a statement saying that the early release occurred because the state prison agency thought the Sacramento and Los Angeles crimes were to be served concurrently instead of consecutively. Roldan, however, said that was not the case; Olson's sentence for the Sacramento crime was overlooked.

And Naj Alikhan, a spokesman for SEIU Local 1000, the union representing the prisons' case records workers, said the system -- not a clerk -- is to blame.

"It really comes down to: There's too little training, there's too little technology," Alikhan said.

"Ultimately, what it comes down to shows the complexity of the prison code."

michael.rothfeld@latimes.com

andrew.blankstein@latimes.com