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 template, the officials say, was created from a secret database of phone call records collected by the spy agency. It has been used since 9/11 to identify calling patterns that indicate possible terrorist activity. Among the patterns examined: flurries of calls to U.S. numbers placed immediately after the domestic caller received a call from Pakistan or Afghanistan, the sources say.
USA TODAY disclosed this month that the NSA secretly collected call records of tens of millions of Americans with the help of three companies: AT&T, Verizon and BellSouth. The call records include information on calls made before the Sept. 11 attacks.
Verizon and BellSouth released statements last week denying they had contracts with the NSA to provide the call information. A Verizon spokesman said the company's statement did not include MCI, the long-distance company that Verizon acquired in January.
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."
"They see everything," says Sergio Nirenberg, director of systems engineering at Science Applications International Corp., a Fortune 500 research and engineering company that works with the federal government. Nirenberg said he does not have direct knowledge of the NSA database.
The disclosure of the call record database has raised concerns among lawmakers, such as Sen. Ron Wyden, D-Ore., that the records give the government access to information about innocent Americans. President Bush has insisted that intelligence efforts are only "focused on links to al-Qaeda and their known affiliates."
The intelligence officials offered new insight into one way the database of calls is used to track terrorism suspects.
The officials, two current U.S. intelligence officials familiar with the program and two former U.S. intelligence officials, agreed to talk on condition of anonymity. The White House and the NSA refused to discuss the template or the program.
Using computer programs, the NSA searches through the database looking for suspicious calling patterns, the officials say. Because of the size of the database, virtually all the analysis is done by computer.
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.
Not every call record contains the same level of detail. Depending upon how a business has its phone system set up, the call detail records might not register complete information on an outgoing call, Nirenberg says.
The records might note only the general number of the business, not the desk extension or, in the case of a hotel, the room extension. Incoming calls that don't go through the switchboard and are dialed directly would have complete call detail records, Nirenberg says.
Not all local calls generate a call detail record, Nirenberg says. But that's not to say that phone companies can't create a record for local calls.
"It's just a matter of whether they enable that function" that allows that to happen, he says. Cellphone calls, on the other hand, create call detail records in almost every case.
Toll calls meaning those that aren't technically long-distance but still cost extra also generate call detail records, he says. "If they charge you separately for it, they have a call detail record," Nirenberg says.
The current and former intelligence officials say that the point of the database is to create leads. The database enables intelligence analysts to focus on a manageable number of suspicious calling patterns, they say. 
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