Washington ? The government must reveal the names of those held in the investigation of the Sept. 11 attacks, a federal judge ruled Friday, rejecting claims terrorists could use the information to plot new crimes.

The Justice Department has not justified a blanket policy of secrecy about more than 1,000 people picked up since the jetliner attacks, U.S. District Judge Gladys Kessler ruled. She gave the government 15 days to provide the names.

The ruling is a major setback for the government, which is only beginning to defend its post-Sept. 11 security and anti-terrorism measures in court. The Justice Department did not immediately say whether it would appeal.

“The government has used its arrest power to detain individuals as part of an investigation that is widespread in its scope and secrecy,” Kessler wrote. “Unquestionably, the public’s interest in learning the identities of those arrested and detained is essential to verifying whether the government is operating within the bounds of the law.”

The judge said there may be exceptions to the release of names, if an individual detainee objects or if the government can show that separate court orders prohibit release of information about someone held as a material witness in a terrorism investigation.

She gave the Justice Department 15 days to back up its claim that it can withhold information about material witnesses. A material witness allegedly has substantial information about a crime but is not himself charged. Such witnesses may be arrested, but may not be held indefinitely.

“The decision is a complete repudiation of the attorney general’s policy of rounding up hundreds of individuals in secrecy,” said Kate Martin, whose Center for National Security Studies is among civil rights and human rights organizations that sued the government for information about the secret arrests.

“The opinion is a vindication of the basic principle that you can’t have secret arrests, that secret arrests are undemocratic.”

Those arrested are all apparently foreign citizens, and many have been charged with immigration violations. Some have apparently been deported.

Kessler largely limited her ruling to the government’s obligations under the federal Freedom of Information Act. In a partial victory for the Justice Department, she rejected the notion that the civil liberties groups have a constitutional free-speech right to some information.

Kessler also agreed with the government that it may keep other details secret. There are valid security reasons for not revealing the dates and location of arrests and detentions, Kessler said.

The judge rejected the government’s national security arguments on another front. There is no reason to keep secret the names of any lawyers representing detainees, she said. Those names must also be made public in 15 days.

Kessler was brusque in dismissing many of the government’s justifications, and noted that prosecutors did not claim or even hint that any of the detainees had connections to terrorism.

The Justice Department was equally tart in response.

The ruling “impedes one of the most important federal law enforcement investigations in history, harms our efforts to bring to justice those responsible for the heinous attacks of Sept. 11, and increases the risk of future terrorist threats to our nation,” said Robert McCallum, assistant attorney general for civil rights. He did not say whether the department would appeal.

The Justice Department has said nearly 1,200 people were swept up by federal, state and local authorities after the Sept. 11 attacks. Most already have been deported.

Apart from periodic updates on the number still held, the Justice Department has tried to keep information about the arrests under wraps.

The government disclosed that between Sept. 11 and June 24, there were 752 arrested or detained in immigration charges. The others were arrested for differing charges.

In late June, the Justice Department reported that at least 147 people still were being held, including 74 on charges involving immigration infractions. Prosecutors have not said how many people are being held as material witnesses.

In June, the Supreme Court blocked a judge from opening immigration hearings for foreign terrorism suspects, granting the Bush administration’s emergency request for a stay.