An internal FBI audit has found the agency violated rules more than 1,000 times while collecting data on US domestic phone calls, e-mails and financial transactions in recent years.
The Washington Post reported today that the number of violations uncovered by the audit was far greater than those previously documented in a Justice Department report in March.
The vast majority of newly discovered violations were instances in which telephone companies and Internet providers gave agents phone and e-mail records the agents did not request and were not authorised to collect.
The agents retained the information in their files, which mostly concerned suspected terrorist or espionage activities, according to the report.
The new audit covers just 10% of the FBI's national security investigations since 2002, so the actual number of violations in the FBI's domestic surveillance efforts probably number several thousand.
Of the more than 1,000 violations uncovered by the new audit, about 700 involved the provision of information by phone companies and other communications firms that exceeded what the FBI's National Security Letters had sought.
However, some two dozen of the newly discovered violations involved agents' requests for information that US law did not allow them to have, the audit found.
National Security Letters allow the FBI to compel the release of private information such as communications or financial records without getting court authority.
Their use has grown since the 11 September 2001 attacks. More than 19,000 such letters were issued in 2005 seeking 47,000 pieces of information.
FBI officials said the audit found no evidence that any agent knowingly or willingly violated the laws or that supervisors encouraged such violations.