"National Security Letters (NSLs) are roughly comparable to administrative subpoenas. They need no prior judicial approval. Intelligence agencies issue them for intelligence gathering purposes to telephone companies, Internet service providers, consumer credit reporting agencies, banks, and other financial institutions, directing the recipients to turn over certain customer records and similar information. The 111th and 112th Congresses saw a number of proposals to amend NSL authority. None were enacted. In the 112th Congress, they were closely associated with the scheduled expiration of various provisions of the Foreign Intelligence Surveillance Act (FISA). In the 113th Congress, they have often been grouped with legislation that addresses the exercise of FISA authority."
CRS Report for Congress, R43322