A hard drive containing sensitive records from the Clinton administration including home addresses and Social Security numbers of high-level officials and their family members has been missing for six months from the National Archives and Records Administration, the agency told Congress on Tuesday.

According to a release from Rep. Darrell Issa (R-Calif.), the ranking member of the Oversight and Government Reform Committee, the missing hard drive includes more than 100,000 Social Security numbers, including that of former Vice President Al Gore’s daughter, contact information for various Clinton administration officials, Secret Service and White House operating procedures, event logs, social gathering logs, political records and other highly-sensitive information.

Issa said NARA Inspector General Paul Brachfeld told committee staff members of the lost data at a closed-door briefing Tuesday and said the extent of the information that was on the missing hard drive is still unknown.

Issa called for NARA’s Acting Archivist Adrienne Thomas to appear at a previously scheduled hearing of the Subcommittee on Information Policy, Census, and National Archives on Thursday, to explain how the hard drive could have gone missing.

“This egregious breach raises significant questions regarding the effectiveness of the security protocols that are in place at the National Archives and Records Administration,— Issa said.

The missing drive contained one terabyte of data, the equivalent of millions of books.

NARA has been working to convert the records on the Clinton administration to digital files. Brachfeld said the hard drive had been moved from a secure area while it was in use.

A NARA spokeswoman did not return an immediate request for comment.