An audit by the inspector general last year found the FBI demanded personal records without official authorization or otherwise collected more data than allowed in dozens of cases between 2003 and 2005. Additionally, last year's audit found that the FBI had underreported to Congress how many national security letters were requested by more than 4,600.

The new audit, which examines use of national security letters issued in 2006, "will identify issues similar to those in the report issued last March," Mueller told senators. The privacy abuse "predates the reforms we now have in place," he said.

On national security letters:

National security letters, as outlined in the USA Patriot Act, are administrative subpoenas used in suspected terrorism and espionage cases. They allow the FBI to require telephone companies, Internet service providers, banks, credit bureaus and other businesses to produce highly personal records about their customers or subscribers without a judge's approval.

More on the abuse of national security letters is here.