From John Diamond and Leslie Cauley’s “Pre-9/11 records help flag suspicious calling” (USA TODAY: 22 May 2006):
Armed with details of billions of telephone calls, the National Security Agency used phone records linked to the Sept. 11, 2001 attacks to create a template of how phone activity among terrorists looks, say current and former intelligence officials who were briefed about the program. …
The “call detail records” are the electronic information that is logged automatically each time a call is initiated. For more than 20 years, local and long-distance companies have used call detail records to figure out how much to charge each other for handling calls and to determine problems with equipment.
In addition to the number from which a call is made, the detail records are packed with information. Also included: the number called; the route a call took to reach its final destination; the time, date and place where a call started and ended; and the duration of the call. The records also note whether the call was placed from a cellphone or from a traditional “land line.” …
Calls coming into the country from Pakistan, Afghanistan or the Middle East, for example, are flagged by NSA computers if they are followed by a flood of calls from the number that received the call to other U.S. numbers.
The spy agency then checks the numbers against databases of phone numbers linked to terrorism, the officials say. Those include numbers found during searches of computers or cellphones that belonged to terrorists.
It is not clear how much terrorist activity, if any, the data collection has helped to find.