ASHINGTON, March 7 - The Supreme Court, in a decision indicating that its re-examination of the role of judges in criminal sentencing is far from over, on Monday overturned a 15-year mandatory sentence that a federal appeals court had ordered imposed on a "career criminal."

The justices said the appeals court had mistakenly ordered the trial court to conduct too wide-ranging an investigation into whether the defendant's previous convictions met the definition of prior offenses that would bring about the mandatory minimum sentence under a federal law, the Armed Career Criminal Act. The trial judge's role should be considerably more limited, the court said in a 5-to-3 opinion by Justice David H. Souter.

The ruling strongly suggested that the sentencing debate, which earlier this year produced a decision that effectively amended the federal sentencing guidelines on the ground that they gave judges too much authority, will now take aim at a different target: prior offenses.

In 2000, when the court ruled in Apprendi v. New Jersey that the Constitution barred judges from making factual findings that led to increased sentences, the justices carved out one exception: findings that related to prior offenses.

Federal statutes and those in many states impose greater, often mandatory, sentences based on a defendant's prior record, so an interpretation of that record can often be the most important part of the sentencing process. Ever since the Apprendi decision, there has been growing tension between the rule the court announced and the one exception it maintained.

The question in Monday's case was whether four burglaries to which Reginald Shepard had earlier pleaded guilty qualified as the kind of prior offenses that made him eligible for the 15-year mandatory minimum sentence imposed on felons who are convicted of possession of firearms.

Under the federal law, a burglary of a house or other structure counted toward the minimum, while a burglary of a car or boat did not.

Mr. Shepard had pleaded guilty to the burglaries, but the plea agreement did not contain the necessary details about the crimes. Consequently, the Federal District Court in Boston refused the government's request for the 15-year mandatory minimum on the ground that the prosecution had not demonstrated that the burglaries were the relevant kind. Mr. Shepard received a three-year sentence instead.

On the government's appeal, the United States Court of Appeals for the First Circuit, also in Boston, instructed the district court to look for further evidence about the burglaries, including examining the original police reports that formed the basis for the prosecutor's complaint. In a subsequent decision, it ordered the district court to impose the mandatory minimum sentence.

In his opinion overturning the appeals court's decision, Justice Souter said the Supreme Court's most relevant precedent, as well as more recent developments - a reference to the Apprendi decision - meant that the trial judge's inquiry should be limited to official court records. In cases involving plea bargains, he said, that meant the judge could look at the transcript of the plea agreement or related findings of fact, but could not go beyond the court files to look at police reports.

In a concurring opinion, Justice Clarence Thomas said the time had come to take up a direct challenge to the Apprendi exception. "Innumerable criminal defendants have been unconstitutionally sentenced under the flawed rule" that the court has maintained, he said.

In a dissenting opinion, Justice Sandra Day O'Connor said the decision, Shepard v. United States, No. 03-9168, made "little sense" and would "substantially frustrate Congress's scheme for punishing repeat violent offenders who violate federal gun laws." Justice O'Connor said that it was a "commonsense inference" that Mr. Shepard's admitted burglaries had occurred in buildings.

Justice O'Connor, whose opinion was joined by Justices Anthony M. Kennedy and Stephen G. Breyer, noted that she had been a dissenter from the Apprendi decision and subsequent rulings that expanded on it. "It is a battle I have lost," she said.

She said the court was now moving "into new territory that Apprendi and succeeding cases had expressly and consistently disclaimed." Justice O'Connor added, "Today's decision reads Apprendi to cast a shadow possibly implicating recidivism determinations, which until now had been safe from such formalism."

Chief Justice William H. Rehnquist did not take part in this case.

There were these other developments at the court on Monday:

Juvenile Death Penalty

The court set aside death sentences for six men on death row in four states for crimes they committed before the age of 18. The actions, certain to be followed by others, were necessitated by the court's ruling last week in Roper v. Simmons that the execution of those who committed crimes as juveniles violated the Eighth Amendment's prohibition against cruel and unusual punishments. Three of the six were from Texas, while the others were from Virginia, Alabama and Mississippi.

Prisoners' Suits

The court ruled by a vote of 8 to 1 that state prisoners challenging parole procedures or other procedural aspects of prison life can use a Reconstruction-era civil rights statute to get into federal court rather than the considerably more restrictive habeas corpus statute. Justice Breyer wrote the opinion in Wilkinson v. Dotson, No. 03-287, and Justice Kennedy was the lone dissenter.

The distinction the court drew Monday, while technical, has considerable practical importance for prisoners. The decision affirmed a ruling by the United States Court of Appeals for the Sixth Circuit, in Cincinnati, which Ohio prison officials had appealed to the Supreme Court.

In order to file a habeas corpus petition, which challenges the constitutionality of a conviction or sentence, a prisoner must first exhaust any procedures the state might offer. But there is no such requirement for going directly to federal court under the Civil Rights Act of 1871, better known as Section 1983. Justice Breyer said that prisoners bringing a challenge that, if successful, would simply get them a hearing should not be forced to use habeas corpus because they were not seeking the relief that habeas corpus provides, namely their freedom.