To gather intelligence, algorithms are used to sift through the communications, and forward on, for storage and possible analysis, those which are believed likely to contain foreign intelligence. The first stage of filtering is done by the telecoms themselves, at the request of the NSA. The filters are not always perfect, and sometimes purely domestic communications are caught. In the event that domestic communications are swept up, the minimization procedures are applied. The procedures require that most "incidental" communications between U.S. persons be destroyed, with a few exceptions. The exceptions include: the content is encrypted, it contains foreign intelligence, or is evidence of a crime. This last exception allows the NSA to pass to the
Drug Enforcement Administration's Special Operations Division (SOD) information relating to drug trafficking. The program has been likened to
Project SHAMROCK, in which the NSA, and its predecessor organization were given daily microfilm copies of all telegrams entering and exiting the United States. == SIGADs ==