The case stems from an FBI request for records of a library patron without identifying the threat posed by the person, said an attorney from the American Civil Liberties Union, representing the unidentified library, in U.S. District Court.

The library chose to challenge the order.

A Justice Department attorney argued the FBI probe would be threatened if the name of the library and other details were disclosed.

"Maybe if we knew all the facts there might be some limited need for the gag," said Ann Beeson, the ACLU attorney. "We believe the government has done nothing to justify the gag."

Enacted after the September 11, 2001, attacks, the Patriot Act allows authorities to search personal records from bookstores, businesses, hospitals and libraries.

The FBI request orders the recipient to keep its request secret. But the ACLU considers the gag order to be a breach of the U.S. Constitution's guaranteed right to freedom of speech.

Beeson said the library was not trying to force the FBI to disclose its entire probe but wants the right to discuss the case publicly when Congress debates this year whether to extend key parts of the Patriot Act.

"Our client wants to appear before Congress and talk to the public and the press why he believes the power of the FBI to demand library records is a bad idea," she said.

The U.S. House of Representatives, ignoring protests from civil liberties groups, voted this summer to reauthorize 16 provisions of the act that expire at the end of the year, including the library clause.

The Senate is expected to take up the matter after lawmakers return from summer recess.

Kevin O'Connor, the Justice Department's attorney, asked U.S. District Judge Janet Hall not to lift the gag order.

"If you do this, the target of this investigation will learn that the government is investigating an organization in which he's involved," he said. "He is likely to stop doing what he is doing."

O'Connor said the FBI wanted information about someone who had used "an electronic communication service provider" on the premises -- an apparent reference to the Internet available at many public libraries.

The judge indicated she would rule next week.