Federal authorities have dropped their demand for records from a library computer, but not without warning the librarians who refused to release them that under other circumstances their failure to cooperate "could have increased the danger of terrorists succeeding."

The FBI said Monday that it has discounted a potential terrorism threat that prompted it to seek records last year from a computer at one of 26 Connecticut libraries that are part of a consortium called the Library Connection.

Four librarians on the consortium's board who received the demand resisted, which the FBI said slowed its work.

"In this case, because the threat ultimately was without merit, that delay came at no cost other than slowing the pace of the investigation," John Miller, the FBI's assistant director, said in a statement. "In another case, where the threat may be real, the delays incurred in this investigation could have increased the danger of terrorists succeeding."

The American Civil Liberties Union, which represented the librarians who received the demand for records, said the librarians might have been willing to comply with a similar demand had it been approved by a judge.

"I'm glad that we're vindicated in resisting the request for the records," said George Christian, of Windsor, Conn., one of the librarians who received a national security letter demanding the records. "We're just protecting our patrons to the extent we can."

The letter sought subscriber and billing information related to a computer used within a 45-minute time period Feb. 15, 2005, when the potential threat was transmitted from a library computer. Authorities have declined to say where exactly that computer was located, but said the request did not involve reading lists of library patrons.

"We concluded that based on the passage of time as well as other information we've been able to develop that this threat is probably not viable," said Connecticut U.S. Attorney Kevin O'Connor, who added that the potential threat had been in an e-mail.

O'Connor said that authorities are trying to prevent attacks and that not every case involves enough information to get a warrant.

The librarians had been under a gag order for months. Last year a federal judge said it unfairly prevented them from participating in a debate over how the Patriot Act should be rewritten, but by the time the FBI dropped its appeal in April, Congress had already voted to reauthorize the law.

"While the government's real motives in this case have been questionable from the beginning, their decision to back down is a victory not just for librarians, but for all Americans who value their privacy," said Ann Beeson, associate legal director of the ACLU.

O'Connor said that the national security letter was appropriately issued and that the ACLU should not question the motives of federal agents trying to investigate a threat.

Prosecutors argued that secrecy in demands for records is necessary to avoid jeopardizing investigations, and that the gag order prevented only the release of librarians' identities, not their ability to speak about the Patriot Act.

The law, initially passed shortly after the Sept. 11, 2001, attacks, allows expanded surveillance of terror suspects, increased use of material witness warrants to hold suspects incommunicado and secret proceedings in immigration cases. It also removed a requirement that any records sought in a terrorism investigation be those of someone under suspicion. Now anyone's records can be obtained if the FBI considers them relevant to a terrorism or spying investigation.