WASHINGTON (CNN) -- The National Archives -- a repository of important government documents, including the U.S. Constitution -- has lost a computer hard drive containing large volumes of Clinton administration records, including the names, phone numbers and Social Security numbers of White House staff members and visitors.

The National Archives has lost a hard drive containing large volumes of Clinton administration records.

Officials at the Archives say they don't know how many confidential records are on the hard drive. But congressional aides briefed on the matter say it contains "more than 100,000" Social Security numbers, including one belonging to a daughter of then-Vice President Al Gore. It also contains Secret Service and White House operating procedures, the staffers said they were told.

The hard drive was last seen in the National Archive's complex in College Park, Maryland, sometime between October of last year and the first week of February. It was discovered missing in late March, prompting a thorough search for the small, 2.5 pound device, the Archives said. When it could not be located, the inspector general's office opened a criminal investigation.

On Wednesday, the Archives announced a $50,000 reward for information leading to its return.

The Archives said no national security information is on the hard drive, nor any original documents. But they said it does contain "personally identifiable information," and they take the loss "very seriously."

U.S. Rep. Darrell Issa, R-California, whose staff was briefed on the matter, said the House will hold a hearing Thursday on the incident.

"If they [the National Archives' staff] can't handle a hard drive that may be sensitive properly, we need to ask the question, will they handle the most secret materials properly?" Issa said.

The Archives Wednesday gave the following account of the disappearance:

Last October, the hard drive was moved from a "secure" storage area to a workspace where it was being used for routine recopying to ensure preservation of the records. But work was halted last year because archivists "wanted to investigate using automated tools to generate inspection reports."

Staffers were moved to other projects until an automated tool was found in mid-March -- that's when they found the hard drive was missing.

The device is described as a two terabyte Western Digital MY BOOK external hard drive, measuring 6.5 x 2.1 x 5.4 inches.

More than 110 4-millimeter tape cartridges were copied onto the hard drive. The records included records from the Clinton Administration Executive Office of the President.

The archives said no original records have been lost, and the Archives has a backup hard drive that will enable them to determine what information is on the missing device.

The inspector general said at least 100 people had access to the area where the hard drive was left unsecured, and that janitors, visitors and others also passed through the area.

The Archives said it is reviewing the data on a copy of the missing hard drive and compiling a list of people whose personal information may be compromised. It will notify individuals of the potential breach and will provide them with a year of credit monitoring, the Archives said.

"Because of the extremely large volume of data on the drive, we do not know yet the number of individuals whose privacy has been affected," the Archives said Wednesday. "As individuals are identified, they will be notified."

The Archives also said it has taken steps to improve security, including both physical control of records and the treatment of personal information.

All About National Archives and Records Administration • Bill Clinton • Al Gore