A federal appeals court today overturned the 12-year prison sentence of Michael Fortier, a central figure in the 1995 Oklahoma City bombing, and ordered a lower court to resentence him based on more lenient guidelines.

Fortier, an associate of convicted bombers Timothy McVeigh and Terry Nichols, pleaded guilty to violating firearms laws and failing to tell authorities of the plot to bomb the Alfred P. Murrah Federal Building. His testimony against the two men was critical to the government's case and crucial to their convictions in the bombing, which killed 168 people.

In its ruling today, a three-judge panel of the U.S. Court of Appeals for the 10th Circuit said the judge who sentenced Fortier erred in using first-degree murder guidelines and instead should have used guidelines for manslaughter. The stricter guidelines, the panel ruled, could be used only if Fortier acted with malice and premeditation, or if his crimes involved felony murder, which they did not.

"All things considered, the District Court should not have applied the first-degree murder guideline," the panel said.

John Russell, a Department of Justice spokesman, said officials there will review the decision and decide whether to ask for a rehearing by the full 10th Circuit court. "We are prepared to go back," he said.

Fortier has been jailed since August 1995 and could be released soon based on today's decision. The sentencing guidelines for manslaughter suggest a term of 41 to 51 months, and that is before any consideration for such things as acceptance of responsibility and aid to the government, credits Fortier probably would receive.

"He's way below time served at that point," said Michael G. McGuire, Fortier's attorney.

No date for a new sentencing hearing has been set.

In the months leading up to the massive explosion in Oklahoma City, Fortier was informed by McVeigh on several occasions of his plans to bomb the federal building and was urged to participate in the plot. Although Fortier declined to take part in the bombing, he did receive and help sell firearms stolen by the two conspirators. The proceeds from those sales allegedly were used to help finance the bombing.

Less than a month after the April 19, 1995, bombing, Fortier and his wife, Lori, agreed to cooperate with the FBI. As part of his deal with the government, Fortier appeared as the star prosecution witness in the McVeigh and Nichols trials, providing critical testimony about their movements and motives.

McVeigh was convicted of murder and conspiracy, was sentenced to death, and is appealing his sentence. Nichols was convicted of involuntary manslaughter and conspiracy and given a life sentence. He is seeking a new trial.