Suppose you -- an innocent and patriotic American -- happened to have used a particular computer in a Connecticut library not so long ago. Here are some of the private details about you that the FBI now knows: What you buy online, what you borrow, where you travel, what you search for and read on the Web and who telephones or emails you in your own home or at your workplace. And because of a Justice Department decision in 2003, all of that information gathered about you no longer is destroyed when investigators determine that you are innocent of any terrorism connections. What's more, because of an executive order issued last month, access to that information about you, now is prey to "state, local and tribal" officials and to "appropriate private sector entities." The order does not define the private sector entities.

Welcome to the USA Patriot Act -- and its most controversial tool: National Security Letters, the device that enables the government to gather all of that information about you. Furthermore, the law keeps it a secret from you that such information has been gathered -- and forbids the person required to hand over that information about you ever to speak of the matter. The Patriot Act originally was adopted by a shaken Congress 45 days after 9/11, and now is up for renewal before it expires Dec. 31. Last week the House and Senate reached a compromise version of some changes to the law, and supporters hoped to win approval of the measure before the Thanksgiving recess. But a bipartisan group of senators and representatives worried about civil liberties managed to stall a final vote and some have threatened to filibuster it until more privacy safeguards are guaranteed.

What's the rumpus? The government says the law's 16 provisions are critical to its ability to protect Americans from terrorism and that failure to renew it endangers the nation. Critics -- including such unlikely allies as former Congressman Bob Barr, a conservative Republican from Georgia and the liberal American Civil Liberties Union -- contend that some aspects of the law are an abuse of power. "As a conservative," Barr says, "I really resent that the burden is on the citizen to show the government has abused power." An ACLU attorney told the Washington Post, "If the government monitors the Web sites that people visit and the books that they read, people will stop visiting disfavored Web sites and stop reading disfavored books." One Justice Department official dismissed civil libertarian concerns as eccentric. And an assistant FBI director, Michael Mason, conceded to the newspaper, "I don't necessarily want somebody knowing what videos I rent or the fact that I like cartoons." But, he said, "If those records are never used against a person, if they're never used to put him in jail, or deprive him of a vote, et cetera, then what's the argument?"

So who's complaining? "There has not been one substantiated allegation of abuse of these lawful intelligence tools," Sen. Pat Roberts, the Kansas Republican who is chairman of the Senate Select Committee on Intelligence, told the Post's Barton Gellman. The Catch 22, of course, is the very secrecy of the National Security Letter searches. As Glenn Fine, the Justice Department's inspector general, acknowledged in testimony to members of the House earlier this year, "We do rely on complaints coming in." But he too sees a problem. "To the extent that people do not know of anything happening to them," he said, "there is an issue about whether they can complain."

Checking the balance: Roberts, defending the need for National Security Letters, has pointed out that the FBI provides "valuable reporting" in semi-annual reports that give "the committee with the information necessary to conduct effective oversight." The reports tell the committee how many times the FBI issued National Security Letters (now estimated by the Post at about 30,000 a year), whether the letters sought financial credit or communications records and how many of the targets were Americans. Some committee members have asked to see a sampling of the letters, a description of information they have gathered and examples of how they have helped particular investigations. The Justice Department has not responded to those requests. Last year, Congress asked for a description of "the scope of each letter" and the "process and standards for approving them." The Justice Department has not responded. "People have to depend on their elected representatives to do the job of oversight they were elected to do," FBI General Counsel Valerie Caproni told the Post. "And we think they are doing a fine job of it."

Test case: The Library Connection, the Connecticut network targeted by the National Security Letter in an FBI hunt for all of the people who used that one library computer, is in federal Appeals Court trying to win the right to challenge the order and make its case to Congress while renewal of the Patriot Act was being debated there. The network risked prosecution by seeking legal advice on how to respond to the letter and initially won the right to go public, but the federal government appealed the lower court ruling and a federal Appeals Court blocked the lower court's decision. In the legal dust-up to keep the letter to the Library Connection a secret, the federal government neglected to censor court documents sufficiently and left the Library Connection's name in plain view, enabling newspapers to publish the library network's identity. The federal government nevertheless is continuing to press its case that the Library Connection's identity must be kept secret so any future showdowns over such secrecy are not undermined.

The reforms: Congressional negotiators seeking to win the law's renewal want to continue authorizing the government to secretly subpoena library and business records -- but in a compromise now would require agents to specify how the request is connected to an investigation. In addition, and the library or business could seek a judicial review. Another change would permit the recipient of a National Security Letter to contact a lawyer and seek a judicial review, "an enormous improvement on the national security letters, says Pennsylvania Republican Sen. Arlen Specter, the powerful chairman of the Judiciary Committee who Friday nevertheless joined the one dozen Senate and House members opposing the revised version of the law. He wants to shorten the number of years until the next review of the Patriot Act from seven years to four years.