California authorities rearrested Sara Jane Olson at noon Saturday, just hours after she was prevented from flying home to Minnesota from Los Angeles, and said she must serve one more year in prison because they miscalculated her release date.

The former member of the radical Symbionese Liberation Army had been paroled Monday from a California women's prison after serving about six years for her role in a 1975 plot to kill Los Angeles police officers by blowing up their patrol cars.

Officials from the California Department of Corrections and Rehabilitation said at a news conference that they had made a mistake in computing the amount of time Olson should serve in a separate case in which she pleaded guilty to second-degree murder for participating in a Sacramento-area bank robbery in which another SLA member killed a customer.

"The department is sensitive to the impact that such an error has had on all involved in this case and sincerely regrets the mistake," Scott Kernan, the agency's chief deputy secretary of adult operations, said at a Saturday afternoon news conference. "The department has launched a full investigation."

Kernan called the case "extremely complicated, given the amount of changes to the sentencing laws that have occurred over the last 30 years."

Olson should have been sentenced to 14 years, not 12, for the two crimes, Kernan said. He said state officials had failed to account for the bank robbery. The earliest possible release date for Olson now is March 17, 2009, he said. At that point, she will have served half of the 14-year term.

Like most California inmates, Olson has earned credit against her sentence for working while in prison. She served on a maintenance crew that swept and cleaned the main yard of the Central California Women's Facility in Chowchilla, according to prison officials.

She was taken to a prison in Chino on Saturday but will be moved back to Chowchilla, Kernan said.

When news organizations reported Olson's release Friday, law enforcement officials reacted with dismay and raised questions about whether she had been let out too early.

Jon Opsahl, son of Myrna Opsahl, the woman killed in the bank robbery, called the Sacramento County district attorney's office and said he believed Olson had not served enough time.

Corrections department officials acknowledged that they began an intensive review of their internal calculations of the sentence after getting questions from the Sacramento County district attorney's office and a local television reporter, but they denied that they had bowed to pressure.

After Olson was taken back into custody, Los Angeles Police Protective League President Tim Sands issued a statement, saying, "We are relieved that Sara Jane Olson has been returned to prison for another year."

But Sands said the organization was "far from satisfied. Parole shouldn't even be an option for terrorists who are convicted of murdering innocent bystanders and attempting to murder police officers. Anyone who tries to kill police officers should get significant jail time and serve their full sentence."

Olson's attorney, Shawn Chapman Holley, said she was outraged by the rearrest and asserted that her client had been illegally arrested and is now being "illegally imprisoned."

Holley said she was surprised to receive a phone call late Friday night from Olson, who told her that law enforcement officials at Los Angeles International Airport "were telling her her travel pass was rescinded and they would escort her back to her mother's home in Palmdale."

After midnight, Holley said, she got another call from Olson, telling her that Olson had been taken to her mother's home in a law enforcement convoy and that although she was not under arrest, law enforcement officials had stationed a car in front of the house and told her she would be followed if she left.

Holley said she planned to file a writ of habeas corpus seeking Olson's release within the next few days.

She scoffed at the suggestion that there had been "a computation error."

"We received an order from the state parole board more than a month ago informing us that she would be released on March 17," Holley said.

She referred to a decision of the board, saying that on Oct. 12, 2007, the panel had notified a Los Angeles Superior Court judge that "it did not intend to impose" a one-year enhancement that had been challenged by Olson's attorneys. The decision went on to say that Olson's "earliest possible release date has been recalculated to March 17, 2008."