The National Archives is sending letters to Clinton administration staff and thousands of people who visited the White House during Bill Clinton’s presidency, warning them that their personal information was compromised after a computer hard drive containing a large amount of sensitive material ­— including Social Security numbers, political records and logs of events — went missing about a year ago.

Paul Equale, founder of Equale & Associates, was one of the more than 250,000 people who will receive similar letters this month, saying that his Social Security number may have been compromised.

“This for me is a little bit like the White House gate-crashers because we always assume that the security at the highest levels of the federal government is going to be the best practices and when we find out that is not the case, it causes us to be concerned about all of our data security as it relates to the government,— Equale said.

Equale met with Clinton administration officials several times as a member of advisory panels and made dozens of other visits to the Oval Office to see the president and other senior officials.

“I took for granted that anytime I was asked for my Social Security number with regard to meeting with the president or going to the Oval Office, or whatever, I never gave it a second thought,— Equale said. “Now I have to give it a second thought.—

The hard drive — part of a storage collection containing snapshots of the contents of computers of former White House employees — was discovered missing around March 2009, during a National Archives project to make logs of the records in the collection.

The security breach also affected one of former Vice President Al Gore’s daughters, whose Social Security number was on the drive. Members of Congress and their staffs also may have had their personal information put in jeopardy if they visited the White House.

“NARA is attempting to inform all individuals whose personally identifiable information was contained on the missing hard drive,— Acting Archivist Adrienne Thomas wrote, according to a copy of the form letter obtained by Roll Call.

“In an effort to determine the nature of the data contained on the hard drive, NARA conducted an initial search of the data contained on a duplicate drive and determined that the missing hard drive contains personally identifiable information, particularly names and social security numbers of former Clinton Administration staff (both within the [Executive Office of the President] and at other federal agencies) as well as individuals who may have visited or contacted the White House during the Clinton Administration.—

The letters are being sent on a “rolling basis, as we identify particular individuals,— according a fact sheet accompanying the letter.

It’s unclear how much more data in the missing drive has yet to be sifted through. The Archives public affairs office did not respond to e-mailed questions regarding the status of the data’s review.

The National Archives and Record Administration inspector general and the U.S. Secret Service have an active investigation into the lost records with a reward of up to $50,000 for information leading to the recovery of the missing hard drive. The IG hasn’t released a report to date on the status of what has been found.

Lawmakers learned that the drive was missing from the Archives facility in College Park, Md., last May. The drive was lost between October 2008 and March 2009 and it contained two terabytes of data, enough to fill millions of books. None of the records are missing because the Archives has a backup copy of the contents of the hard drive.

This is the second time the Archives has sent letters to individuals whose information was compromised. Previously, the Archives authorized the credit bureau Experian to send 26,449 letters to affected people.