WASHINGTON — The Supreme Court said Wednesday that states may not require people who appear before a grand jury to keep their testimony a secret forever.

The court ruled unanimously that such a Florida law, similar to those in 15 other states, violated free-speech rights.

The invalidated Florida law barred grand jury witnesses from disclosing their testimony even after the investigation ended.

The justices were told that Alabama, Arizona, Indiana, Kentucky, Louisiana, Michigan, Mississippi, Missouri, Nevada, North Carolina, North Dakota, South Dakota, Texas, Utah and Washington have similarly restrictive laws.

The federal government and most states impose no disclosure restrictions on grand jury witnesses, apparently not even while an investigation is in progress.

But the new decision does not affect state laws that bar people from disclosing their grand jury testimony while the secret panel’s investigation is active.

Grand juries are presented evidence by prosecutors who seek to have charges filed against criminal suspects. It is up to the grand jury to decide whether charges should be filed and its proceedings are conducted in secret to spare the reputations of presumably innocent people.

“The interests advanced by the portion of the Florida statute struck down are not sufficient to overcome (a witness’) First Amendment right to make a truthful statement of information” lawfully obtained, Chief Justice William H. Rehnquist wrote for the court.

“The ban extends not merely to the life of the grand jury but into the indefinite future,” he said.

Jane Kirtley of the Reporters Committee for the Freedom of the Press said the decision will help journalists who want to interview grand jury witnesses.

“The court recognized that this kind of statute is an overwhelming restraint that could not withstand constitutional scrutiny,” she said.

Michael Smith, a former reporter for the Charlotte Herald-News in Charlotte County, Fla., wants to write a news story and possibly a book about a 1986 grand jury investigation into allegations of corruption in the county sheriff’s department and the local state attorney’s office.

He testified before that grand jury, which ended its investigation in 1986 and issued no indictments.

Smith, now a National Guard sergeant at Fort Indiantown Gap in Annville, Pa., was called by the county grand jury to testify about stories he had written about the alleged corruption.

“The grand jury itself is kind of an inquisition process,” he said Wednesday. “If witnesses aren’t allowed to say what they did or heard in there, then you not only have an inquisition but an inquisition operating in the dark.”

George Waas, the assistant attorney general who handled the case, said the decision will clear up any confusion about the validity of such open-ended secrecy laws.

“Our opinion is that it’s an issue that needed to be clarified, and now it has been,” he said.

Smith challenged the Florida secrecy law because he feared he could be prosecuted for revealing his grand jury testimony.