WASHINGTON — The FBI improperly used national security letters in 2006 to obtain personal data on Americans during terror and spy investigations, Director Robert Mueller said today.

Mr. Mueller told the Senate Judiciary Committee that the privacy breach by FBI agents and lawyers occurred a year before the bureau enacted sweeping new reforms to prevent future lapses.

Details on the abuses will be outlined in the coming days in a report by the Justice Department’s inspector general.

The report is a follow-up to an audit by the inspector general a year ago that found the FBI demanded personal data on people from banks, telephone, and Internet providers and credit bureaus without official authorization and in non-emergency circumstances between 2003 and 2005.

Mr. Mueller, noting senators’ concerns about Americans’ civil and privacy rights, said the new report “will identify issues similar to those in the report issued last March.” The similarities, he said, are because the time period of the two studies “predates the reforms we now have in place.”

He added: “We are committed to ensuring that we not only get this right, but maintain the vital trust of the American people.”

Mr. Mueller offered no additional details. Several other Justice Department and FBI officials familiar with this year’s findings have said privately the upcoming report will show the letters were wrongly used at a similar rate as during the previous three years.