The Department of Justice says it secretly sought phone records and other documents of 3,501 people last year under a provision of the Patriot Act that does not require judicial oversight.
The records were obtained with the use of what are known as National Security Letters, which can be signed by an FBI agent and are only for use in terrorism cases.
The letters require telephone companies to keep secret even the existence of the request for records.
Assistant Attorney General William Moschella told Congress last month that 9,254 National Security Letters were issued in 2005 involving 3,501 people.
Federal law enforcement sources say the National Security Letters are being used to obtain phone records of reporters at ABC News and elsewhere in an attempt to learn confidential sources who may have provided classified information in violation of the law.
The FBI says its request for reporters' phone records are made in compliance with the law.