National Security Letters are administrative orders compel their recipients to provide information to federal investigators. As authorized by the USA PATRIOT Act, the letters included gag orders that forebade recipients from discussing the letter's contents and instructions, the letters did not require a judge's authorization, and were subject to only limited judicial review. These letters were used by the Federal Bureau of Investigation, and possibly other federal agencies.
In Doe v. Mukasey, 549 F.3d 861 (2d Cir. 2008), the Second Circuit held that the letters' gag orders and lack of judicial review violated the First Amendment.