Exigency letters have been used extensively by the FBI to demand information about US citizens from telecom companies. According to Glenn Greenwald, these letters assure the telecom companies that "(a) the records were needed immediately due to "exigent circumstances" and (b) a subpoena for the records had been submitted to the U.S. Attorneys Office and was in the process of being finalized. "
There is no basis in law for these letters, and a report by the Justice Department's Inspector General found that they had been abused by the FBI over several years. FBI Director Mueller said that he has banned the future use of these letters.