The FBI came calling in Windsor, Conn., this summer with a document marked for delivery by hand. On Matianuk Avenue, across from the tennis courts, two special agents found their man. They gave George Christian the letter, which warned him to tell no one, ever, what it said.

Under the shield and stars of the FBI crest, the letter directed Christian to surrender "all subscriber information, billing information and access logs of any person" who used a specific computer at a library branch some distance away. Christian, who manages digital records for three dozen Connecticut libraries, said in an affidavit that he configures his system for privacy. But the vendors of the software he operates said their databases can reveal the Websites that visitors browse, the e-mail accounts they open and the books they borrow.

Christian refused to hand over those records, and his employer, Library Connection Inc., filed suit for the right to protest the FBI demand in public. The Washington Post established their identities - still under seal in the U.S. Court of Appeals for the 2nd Circuit - by comparing unsealed portions of the file with public records and information gleaned from people who had no knowledge of the FBI demand.

The Connecticut case affords a rare glimpse of an exponentially growing practice of domestic surveillance under the USA Patriot Act, which marked its fourth anniversary on Oct. 26. "National security letters," created in the 1970s for espionage and terrorism investigations, originated as narrow exceptions in consumer privacy law, enabling the FBI to review in secret the customer records of suspected foreign agents. The Patriot Act, and Bush administration guidelines for its use, transformed those letters by permitting clandestine scrutiny of U.S. residents and visitors who are not alleged to be terrorists or spies.

The FBI now issues more than 30,000 national security letters a year, according to government sources, a hundredfold increase over historic norms. The letters - one of which can be used to sweep up the records of many people - are extending the bureau's reach as never before into the telephone calls, correspondence and financial lives of ordinary Americans.