Mueller pledged to the Senate Judiciary Committee that he would curb future lapses through broad changes enacted after the FBI was alerted to the privacy breach in March 2007.
"We are committed to ensuring that we not only get this right, but maintain the vital trust of the American people," Mueller said.
He said the upcoming audit by the Justice Department's inspector general, Glenn A. Fine, "will identify issues similar" to those in the earlier report. The abuse, he said, "predates the reforms we now have in place."
He offered no additional details about the audit, expected to be released as early as next week.
Last year's report by Fine's office 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, that audit found that the FBI had underreported to Congress how many national security letters were requested by more than 4,600.
The 2007 report blamed agent error and shoddy record-keeping for the bulk of the problems. It did not find any indication of criminal misconduct. It concluded that many lapses were caused by banks, telecommunication companies and other private businesses giving the FBI more personal client data than was requested.
Several Justice Department and FBI officials familiar with the upcoming findings for 2006 have said privately the audit will show national security letters were used incorrectly at a similar rate as during the previous three years. They said Fine's report credits the FBI with putting the additional checks in place to make sure privacy rights are not violated.
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 court approval.
The number of national security letters issued by the FBI skyrocketed in the years after the Patriot Act became law in 2001, according to last year's report. Fine's annual review is required by Congress, over the objections of the Bush administration.
For 2005, for example, Fine's office found more than 1,000 violations within 19,000 FBI requests to obtain 47,000 records. Each letter issued may contain several requests.
In contrast to the strong concerns expressed by Congress and civil liberties groups after last year's inspector general's report was issued, Mueller's disclosure drew no criticism from senators during just over two hours of testimony Wednesday.
Speaking before the FBI chief, the Senate Judiciary Committee chairman urged Mueller to be more vigilant in correcting what he called "widespread illegal and improper use of national security letters."
"Everybody wants to stop terrorists. But we also, though, as Americans, we believe in our privacy rights and we want those protected," said Sen. Patrick Leahy, D-Vt. "There has to be a better chain of command for this. You cannot just have an FBI agent who decides he'd like to obtain Americans' records, bank records or anything else and do it just because they want to."
Following last year's audit, the Justice Department enacted guidelines that reminded FBI agents to follow the rules governing national security letters. The new rules caution agents to review all data before it is transferred into FBI databases to make sure that only the information specifically requested is used.
Critics seized on Mueller's testimony as proof that a judge should sign off on the national security letters before they are issued.
"The credibility factor shows there needs to be outside oversight," said former FBI agent Michael German, now a national security adviser for the American Civil Liberties Union. He also cast doubt on the FBI's changes.
"There were guidelines before, and there were laws before, and the FBI violated those laws," German said. "And the idea that new guidelines would make a difference, I think cuts against rationality."
