WASHINGTON — The Supreme Court, substantially narrowing the Freedom of Information Act, ruled Monday that the government may keep secret the information it gathers during routine business if the information later becomes part of a criminal investigation.

Until now, government audits and routine reports had to be disclosed upon request. But after Monday's 6-3 ruling, these reports and documents will be off limits to disclosure as soon as a federal prosecutor or investigator displays interest in them.

At the same time, the ruling protects criminal investigations by allowing authorities to hide the details of a pending prosecution from possible suspects.

The court majority, saying it sought a "workable balance" between the interest in public disclosure and the needs of prosecutors, came down squarely on the side of prosecutors. The three dissenters complained that the majority had rewritten the disclosure law in the process.

In a typical criminal probe, officials keep their case secret until it goes to trial. The Justice Department argued that criminal suspects should not be permitted to use the Freedom of Information Act as an end run to learn about a pending investigation.

Ironically, this Freedom of Information Act case was labeled by the court as "John Doe Agency and John Doe Government Agency vs. John Doe Corp., 88-1083." But thanks to lower-court records, the parties were well-known. They were the Defense Contract Audit Agency and the Justice Department vs. the Grumman Corp., a major defense contractor.

In 1978, the defense audit agency, the Pentagon's accounting arm, conducted a routine audit of Grumman. The agency and Grumman disagreed on how some costs should be allocated but no action was taken.

Seven years later, however, the U.S. attorney in New York began a criminal fraud investigation of Grumman. Lawyers for the company then submitted a request under the Freedom of Information Act for documents "related in any way" to the 1978 audit. The defense agency instead sent all the documents to the FBI.

Grumman sued the defense agency, and the U.S. 2nd Circuit Court of Appeals said the agency had to turn over the requested information. Although the law allows the government to withhold "records or information compiled for law enforcement purposes," the court held that the 1978 audit was not compiled for such purposes.

The Justice Department appealed, contending that this decision could seriously undercut federal criminal investigations.