When FBI agents attempted to build cases against foreign nationals accused of terrorism and espionage in the early 1980s, a privacy law barred them from obtaining suspects' U.S. banking records. Congress had passed the Right to Financial Privacy Act (RFPA) in 1978, creating statutory Fourth Amendment protection for an individual's banking records, and inadvertently blocking important investigations.

In response to the bureau's complaints, Congress passed the first National Security Letter statute as an amendment to the RFPA in 1986. The law change allowed the FBI to demand information from financial institutions using administrative subpoenas -- without court order -- if the agency possessed: "specific and articulable facts giving reason to believe that the customer or entity whose records are sought is a foreign power or an agent of a foreign power."

To keep investigations covert, Congress allowed a gag order provision, prohibiting institutions from disclosing receipt of the NSL. Only senior FBI officials could authorize use of the letters.

As the horrendous reality of the Sept. 11 terrorist attacks began to sink into the American consciousness, Congress rushed to provide security agencies with the necessary tools to protect the country, and passed the Patriot Act only 45 days after the attack. Buried deep inside the bill, a handful of amendments greatly increased the power of NSLs.

One of the catalysts for expanding NSLs' domain was their stealth. "The national security letter is an important tool," former Attorney General John Ashcroft told FRONTLINE. "It's important in substantial measure because you don't want to alert individuals who are involved in certain kinds of activity that you are looking at their activity."

The Patriot Act's architects wanted to maximize the value of the gag order, and thus made two key revisions to the NSL statutes: First, they replaced the necessary requirement of "specific and articulable facts … of a foreign power" with a much lesser standard of "relevant to an authorized investigation to protect against international terrorism or clandestine intelligence activities." Second, they expanded NSL issuing authority from FBI headquarters to the supervisors of FBI field offices.

This remodeled NSL authority granted the FBI unprecedented access into Americans' financial and communication records. To combat terror, NSLs could now probe deeper into a larger number of private databases, all in complete secrecy.

In 2000, 8,500 NSLs were issued for records of foreign nationals. Four years later that number exploded to 56,507, with over half pertaining to investigations of U.S. persons. Peter Swire, the privacy counsel to the Clinton White House, said the Patriot Act broadened the use of NSLs, moving from an individual warrant to generalized suspicion. "Instead of getting the suspect's records you get the whole database," he explained.

One database the FBI went after was the central hub of Connecticut's 26 independently operated public libraries. In May 2005 the FBI discovered that information about a potential terrorist threat had been sent from a public computer inside the library system. Investigators produced NSLs and obtained access to every record on every person within the massive Connecticut mainframe.

Four infuriated librarians decided to take action and joined a legal battle between the American Civil Liberties Union (ACLU) and the U.S. government that had begun almost a year before. In the case, an Internet service provider based in New York had received an NSL and approached the ACLU with fears that compliance would jeopardize its clientele's private communications. The ACLU subsequently challenged the NSL and its gag order on the grounds of First and Fourth Amendment violations.

For months it was unclear if the case would ever reach trial, as the simple act of disclosure was a criminal offense for the ISP. But a New York judge ruled the non-disclosure provision unconstitutional in September 2004. In his opinion, U.S. District Court Judge Victor Marrero said, "democracy abhors undue secrecy." As the government filed for appeal, the ACLU incorporated the librarians' complaints into its case, and the ruling has since been upheld. As a result, the Patriot Reauthorization Act of 2005 revised the NSL statutes, allowing recipients to acknowledge receipt when seeking legal advice, and requiring the FBI to certify that disclosure would harm national security. However, the law directed judges to treat the FBI statements "as conclusive unless the court finds that the certification was made in bad faith."

Compelled by the court's rulings, Congress demanded increased oversight and periodic review by the Justice Department. In spring 2007, after investigating the issuance of 293 NSLs, Glenn Fine, the Justice Department's inspector general, discovered 22 possible breaches of governmental policy and 26 additional violations. The 200-page review described a multitude of mistakes across the bureau's entire administrative hierarchy, from improper authorization and requests, to unauthorized record collection. When extrapolated to the nearly 150,000 NSLs issued between 2003 and 2005, Fine believed the small sample's results signaled thousands of potential errors.

Delivering his review before the Senate Judiciary Committee, Inspector Fine equated the bureau's errors to carelessness, confusion, and inadequate training, guidance and oversight. "We did not find that FBI agents sought to intentionally misuse the national security letters," Fine said, "Yet, I do not believe that any of these observations excuse the FBI's widespread and serious misuse of its national security letter authorities."

Many insiders were not surprised. "You want your intelligence agent to go full-bore," Swire told FRONTLINE, "and you want the checks and balances on that so we get energy and we get rule of law."

In response to the Justice Department's report, FBI Director Robert Mueller acknowledged the bureau's errors and vowed to take the necessary steps to stop abuses. "The FBI is acutely aware that we cannot protect against threats at the expense of civil liberties," Mueller said, "We are judged not just by our ability to defend the nation from terrorist attacks but also our commitment to defend the rights and freedoms we all enjoy."

Update: In September 2007, Judge Marrero ruled that despite the 2005 revisions to the Patriot Act, the secrecy provisions surrounding the NSLs violated the First Amendment and the separation of powers guarantee, and declared them unconstitutional. He described the provisions limiting judicial oversight as "the legislative equivalent of breaking and entering, with an ominous free pass to the hijacking of constitutional values." According to The Washington Post, Judge Marrero's ruling will likely "eliminate or sharply curtail" the FBI's use of NSLs; however he delayed enforcement of his ruling for 90 days in order to give the government a chance to appeal.

Fritz Kramer is the production assistant for Spying on the Home Front.