The Supreme Court overturned the 2002 criminal conviction of Enron Corp.'s accounting firm yesterday, nullifying with a single stroke one of the government's biggest victories in the corporate scandals that climaxed the bull market of the 1990s.

The court ruled unanimously that the Houston jury that found Arthur Andersen LLP guilty of obstruction of justice was given overly broad instructions by the federal judge who presided at the trial.

As a result of the faulty instructions, the justices ruled, the firm was convicted without proof that its shredding of documents was deliberately intended to undermine a looming Securities and Exchange Commission inquiry in fall 2001. U.S. District Judge Melinda Harmon should have instructed the jury that the law required the government to prove that Andersen knew it was breaking the law, the court ruled.

"Indeed, it is striking how little culpability the [judge's] instructions required," Chief Justice William H. Rehnquist wrote in the opinion for the court. "For example, the jury was told that, 'even if [Andersen] honestly and sincerely believed that its conduct was lawful, you may find [it] guilty.' " Legal analysts said the decision was a major setback to the Justice Department's corporate crime prosecutions.

"To lose a case like this is huge," said William B. Mateja, a former official of the Justice Department's corporate fraud task force. "Arthur Andersen was the poster-child case of all the corporate fraud cases."

More broadly, some lawyers said the court's decision shows its sympathy for corporate America's view that companies should be freer to engage in routine document destruction -- often under the ironic title of "document retention policy."

That is important because the statute under which the Justice Department prosecuted Andersen was amended by Congress in the 2002 Sarbanes-Oxley law to make it easier for the government to prosecute wrongful document destruction.

"The Supreme Court may be using this as a vehicle to signal some concern" about Sarbanes-Oxley, said Henry T.C. Hu, a professor of corporate and securities law at the University of Texas.

But Mateja, now in private practice, said that Congress's intent to prevent improper document destruction was clear. "I'm still going to counsel clients to be extremely careful if and when they dust off document-retention policies," he said.

Although a rebuke to the government, the court's decision is little comfort for Andersen and its former employees. The Chicago-based firm has a staff of only 200 left out of the 28,000 people who once worked there. But the company said the ruling may help the firm in its main remaining task: fighting shareholder lawsuits related to its work for Enron, Global Crossing Ltd. and other clients.

"We pursued an appeal of this case not because we believed Arthur Andersen could be restored to its previous position, but because we had an obligation to set the record straight and clear the good name of the 28,000 innocent people who lost their jobs at the time of the indictment and tens of thousands of Andersen alumni, as well as to help secure a fair resolution of the civil litigation facing the firm," company spokesman Patrick Dorton said in a statement.

Acting Assistant Attorney General John C. Richter said in a statement that the Justice Department had charged Andersen because of its "determination that the substantial destruction of documents in anticipation of an investigation by the Securities and Exchange Commission violated the law."

Richter said prosecutors continue to examine the decision and will "determine whether to retry the case."

But legal analysts said that is unlikely, given the constitutional prohibition against double jeopardy and the tougher standard of proof required by yesterday's decision.

"It would be 10 times harder with a jury instruction that you must show evil intent," said Paul Kamenar, senior executive counsel for the conservative Washington Legal Foundation, which supported Andersen in the case.

One possible beneficiary of the decision is former investment banker Frank P. Quattrone, who was convicted last year on similar charges of ordering document destruction to thwart investigations in 2000.

It may also affect the conviction of David B. Duncan, the Andersen partner who pleaded guilty to a single count of obstructing justice in April 2002, under the government's interpretation of the law, then testified against his former firm. Duncan is a likely witness in upcoming Enron criminal trials, and he will not be sentenced until his government cooperation has ended.

Andersen was in charge of auditing the books at Enron, the high-flying Houston energy conglomerate whose financial meltdown in fall 2001 wiped out the savings of thousands of employees and other small investors -- and politically damaged the Bush administration, with which Enron Chairman Kenneth L. Lay had been close.

As Enron's collapse became public, Nancy Temple, a lawyer for Andersen, sent an e-mail on Oct. 19, 2001, reminding employees of the company's policy of routine document shredding. Two tons of documents were destroyed until the SEC formally notified Andersen on Nov. 9, 2001, that it was under investigation.

In prosecuting Andersen, the government argued that the shredding was done to prevent the SEC from finding out about such matters as Andersen's role in helping Enron puff up the reported returns of "off balance sheet" activities by units known as "Raptors."

Thus, the government argued, the firm violated a federal law that made it a crime to "corruptly persuade" anyone to cover up evidence. At oral argument in the Supreme Court, a Justice Department lawyer likened Andersen to a felon wiping his fingerprints at a crime scene.

But in his opinion yesterday, Rehnquist suggested that the company was more akin to "a mother who suggests to her son that he invoke his right against compelled self-incrimination."

It "is not inherently malign" to persuade someone to withhold documents from the government, but that is what the government asserted with respect to Andersen's conduct, Rehnquist wrote.

The case is Arthur Andersen LLP v. United States, No. 04-368.

Staff writer Carrie Johnson contributed to this report.

The ruling faulted District Judge Melinda Harmon's instructions to the jury.Chicago-based Arthur Andersen LLP employed 28,000 people before the Enron Corp. scandal became public. "We had an obligation to set the record straight," a spokesman said of the firm's Supreme Court appeal.