National Security Letters in Foreign Intelligence Investigations: Legal Background [Updated July 30, 2015] [open pdf - 1MB]
From the Introduction: "Five statutory provisions vest government agencies responsible for certain foreign intelligence investigations (principally the Federal Bureau of Investigation (FBI)) with authority to issue written commands comparable to administrative subpoenas. A National Security Letter (NSL) seeks customer and consumer transaction information in national security investigations from communications providers, financial institutions, and credit agencies. Over the years, Congress has struggled with efforts to ensure the effectiveness of the NSL authority, while guarding against its abuse."
CRS Report for Congress, RL33320
Congressional Research Service: https://crsreports.congress.gov/