The bureau's New Haven, Conn., field office wrote to the libraries' attorneys at the ACLU, saying it "will not seek to enforce the national security letter delivered to your client, Library Connection, Inc., by FBI personnel on or about July 12, 2005."

A statement from FBI Headquarters in Washington late Monday said the effort was being abandoned because the investigation it was part of had been closed.

The libraries had challenged the constitutionality of the letter before the U.S. district court in Connecticut, saying it -- and the secrecy imposed on its recipients -- violated the First and Fourth Amendments.

"We pursued this matter because librarians should protect the privacy of our patrons," George Christian, executive director of Library Connection, said Monday. "Everyone has the responsibility to make sure the government plays by the rules."

The field office also said the FBI was abandoning its efforts to keep the contents of the national security letter secret, and the ACLU posted the document for the first time.

The climb-down is the latest twist in the troubled history of national security letters -- first introduced in 1986 for use against foreign intelligence agents; greatly expanded after the Sept. 11 attacks by the suite of anti-terror laws called the USA PATRIOT Act; and most recently amended by Congress to ensure they could be challenged in court.

The letters, which can be issued by the FBI without the need for court approval, can demand a range of telecommunications and financial records from banks, credit companies and telephone companies or internet service providers, so long as they are seen as "relevant" to a terrorism or foreign intelligence investigation.

The FBI said that the Library Connection, a consortium of Connecticut public libraries that provides a shared computer infrastructure, was in effect an internet service provider.

The letter sought "any and all subscriber information, billing information and access logs of any person or entity related to" a particular computer between 4 pm and 4.45 pm on Feb. 2, 2005." It is the first time the exact nature of the request has been identified.

In a statement emailed to reporters late Monday afternoon, FBI spokesman John Miller said that "information about a potential terrorist threat" had been sent from that computer at that time.

"Under the law in effect in June of 2005, the national security letter was the most logical prescribed investigative tool to use in seeking this type of information in a counter-terrorism investigation," he said, adding that the information "would have helped the FBI more efficiently investigate and evaluate" the threat.

He said that the investigation had eventually discounted the threat and been closed.

"The information sought had nothing to do with obtaining reading lists of library patrons or any other information that would be reasonably considered intrusive upon any individual's rights as a library patron," concluded Miller.

The ACLU's lead attorney in the case, Ann Beeson, rejected the suggestion that the relatively narrow character of the request was reassuring.

"The request on its face is broad," she said, adding that it could have encompassed any Web sites visited or library catalogue look-ups by any user of the library who happened to sit at that computer during that time.

She said if there was a genuine investigative need for the records, the FBI could have easily sought a subpoena or a warrant.

"Is it a fishing expedition?" she asked. "The answer is we don't know, because this request has never been considered by a court."

Christian said he was disappointed that the case had not gone the distance. "In some ways I would rather have duked it out in court and got a real result," he said.

The law governing national security letters also contains a gag order prohibiting the recipients of a letter from disclosing or discussing its contents.

In May, a U.S. appeals court effectively lifted the gag in the Connecticut case, freeing the librarians to speak out about the case for the first time -- two months after the PATRIOT Act had been reauthorized, and the national security letter provision amended in response to concerns that it might be used in broad "fishing expeditions," and that its use against libraries would chill Americans' free speech rights.

Christian told United Press International that while he was glad the case was over, he was disappointed that he and his fellow litigants had been unable to testify before congress.

"They only heard one side of the story," he said of lawmakers. Partly as a result, he and other critics believe the changes made to national security letters in the reauthorization legislation are, in Christian's words, "a mixed bag."

Although the law now specifies that national security letters can be disclosed to the recipients' attorneys, and can be challenged in court, the other gag provisions -- relating to public disclosure of the letter -- were "now absolute," said Christian.

Under the new law, the gag is for one year, but can be renewed annually at the request of the Justice Department, for an indefinite period.

But the gag cuts both ways, some officials say, pointing out that it also barred the FBI and Justice department from disclosing the narrow character of the request.

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