The retired CIA inspector general who quit as the director of a congressional probe into intelligence failures to prevent the Sept. 11 attacks was asked to resign because he hired a staffer who had been investigated for "security issues," congressional sources said Tuesday.

L. Britt Snider was asked to step down last week by Senate Intelligence Committee Chairman Bob Graham, D-Fla., after Graham learned that Snider had hired a staffer for the committee investigation who had been investigated at another job for mishandling intelligence information. The name of the staffer is not known.

Sources said Snider was aware that the staffer that he hired had "security issues" and had been investigated, but that Snider did not tell anyone else on the committee.

It is unclear how Snider's departure will affect the investigation into the intelligence community's structure, funding, and response to the attack, and whether it left the United States vulnerable to future terror attacks.

The joint panel was expected to conduct hearings as early as next month.

Snider had been on the job for little more than two months. He was hired in February with great fanfare, and was praised by committee leaders for his objectivity.

Some critics, however, suggested Snider's long working relationship with CIA Director George J. Tenet meant the investigation would amount to a whitewash.

Sources said ranking member Sen. Richard Shelby, R-Ala., was told about the controversy and accepted the decision to ask Snider to resign.

The decision was also made with the consideration of the House Intelligence Committee Chairman Porter Goss, R-Fla., and ranking Democrat Nancy Pelosi, D-Calif.

Pelosi said Tuesday that Snider had done a great deal to establish the joint inquiry.

"He will be missed, but he laid a strong foundation for the inquiry by assembling a first-rate staff," she said in a statement. "I am confident they will be able to meet the difficult challenges ahead."

Snider, a Vietnam veteran and a Democrat, was unavailable for comment. His chief deputy, Rick Cinquegrana, who worked for Snider at the Central Intelligence Agency, has been named acting director.

The investigating commission has about 30 staff members, four of whom are working full-time at the CIA gathering information about the Sept. 11 attacks, officials close to the investigation said.

Fox News' Carl Cameron and the Associated Press contributed to this report.