The National Security Letter provision of the Patriot Act radically expanded the FBI's authority to demand personal customer records from Internet Service Providers, financial institutions and credit companies without prior court approval.

Through NSLs the FBI can compile vast dossiers about innocent people and obtain sensitive information such as the web sites a person visits, a list of e-mail addresses with which a person has corresponded, or even unmask the identity of a person who has posted anonymous speech on a political website. The provision also allows the FBI to forbid or "gag" anyone who receives an NSL from telling anyone about the record demand. Since the Patriot Act was authorized in 2001, further relaxing restrictions on the FBI's use of the power, the number of NSLs issued has seen an astronomical increase. The Justice Department's Inspector General has reported that between 2003 and 2006, the FBI issued nearly 200,000 NSLs. The inspector General has also found serious FBI abuses of the NSL power.

The ACLU has challenged this Patriot Act statute in court in three cases.

The first such lawsuit, Doe v. Holder, which resulted in numerous court rulings finding parts of the NSL statute unconstitutional, was settled in August 2010. As a result, the "John Doe" client, Nicholas Merrill, is finally able to publicly identify himself and his former company as the plaintiffs in the case.

Library Connection v. Gonzales, involved an NSL served on a consortium of libraries in Connecticut. In September 2006, a federal district court ruled that the gag on the librarians violated the First Amendment and the government ultimately withdrew both the gag and its demand for records.

Internet Archive v. Mukasey, involved an NSL served on a digital library. In April 2008, the FBI withdrew the NSL and the gag a part of the settlement of a legal challenge brought by the ACLU and the Electronic Frontier Foundation.

In addition, the ACLU has filed a number of Freedom of Information Act (FOIA) request to learn more about the government's use of NSLs.