Via: Wikipedia —
A national security letter (NSL) is a demand letter, which differs from a subpoena. It can be used by US government agencies, mainly the FBI, when investigating matters related to national security. It is issued to a particular entity or organization to turn over various record and data pertaining to individuals. NSLs can only request non-content information, such as transactional records, phone numbers dialed or email addresses mailed to and from.
They also contain a gag order, preventing the recipient of the letter from disclosing that the letter was ever issued. The gag order was ruled unconstitutional as an infringement of free speech in the Doe v. Gonzales case, but this decision was superseded by the Second Circuit Court after the USA PATRIOT Improvement and Reauthorization Act gave recipients of NSL gag orders recourse in court.
On March 14, 2013, Judge Susan Illston of Federal District Court in San Francisco struck down the law establishing NSLs, writing that the prohibition on disclosure of receipt of such an order made the statute “impermissibly overbroad” under the First Amendment. Judge Illston’s ruling also struck down a statute prohibiting legal challenges by recipients of the security letters, but stayed implementation of her ruling to allow the government to appeal the decision. From 2003 to 2006 the Federal Bureau of Investigation issued 192,499 national security letter requests.
Read more at Electronic Freedom Foundation