Testifying Wednesday before the Senate Judiciary Committee, FBI Director Robert Mueller said the inspector general will release the audit of the FBI's use of national security letters in 2006. The agency has been criticized for its use of the anti-terror investigatory tactic, which a federal judge has barred as unconstitutional.

The letters are non-negotiable demands for personal information in the interest of national security and are not reviewed in advance by a judge. Since coming under fire for using the letters, Mueller met with privacy and civil liberty groups and strengthened internal controls.

"(After) the Department of Justice IG review of the use of national security letters, we have instituted new procedures and internal oversight mechanisms, to ensure that we as an organization minimize the chance of future lapses," Mueller said.

He said the report's audit of the FBI's use of national security letters "will identify issues similar" to questions raised last year because "it covers a time period which pre-dates the reforms we now have in place."