The former top aide to Attorney General Alberto Gonzales agreed Friday to testify at a Senate inquiry next week into the firings of eight U.S. attorneys last year.

Kyle Sampson, who resigned two weeks ago amid the furor over the dismissals, will appear next Thursday at a hearing of the Senate Judiciary Committee, his attorney said. His appearance will mark the first congressional testimony by a Justice Department aide since the release of thousands of documents that show the firings were orchestrated, in part, by the White House.

Sampson "looks forward to answering the committee's questions," wrote his attorney, Brad Berenson, in a two-paragraph letter to Committee chairman Patrick Leahy, D-Vt., and the panel's top Republican, Arlen Specter of Pennsylvania.

"We trust that his decision to do so will satisfy the need of the Congress to obtain information from him concerning the requested resignations of the United States attorneys," Berenson wrote.

E-mails between the White House and the Justice Department, dating back to the weeks immediately after the 2004 presidential election, show Sampson was heavily engaged in deciding how many prosecutors would be replaced, and which ones. The Bush administration maintains the dismissals of the eight political appointees were proper.

Democrats, however, question whether the eight were selected because they were not seen as, in Sampson's words, "loyal Bushies."

"He was right at the center of things," Sen. Chuck Schumer, D-N.Y., who is leading the inquiry into the firings, said of Sampson. "He has said publicly that what others have said is not how it happened. ... He contradicts DOJ."

Schumer said he hoped Sampson would provide more detail about who initiated the firings and whether they were politically motivated.

Sampson's agreement to testify next week came a few hours after Sen. John Cornyn, R-Texas, summoned White House counsel Fred Fielding to Capitol Hill to discuss the dispute over whether and under what conditions Bush's top aides will tell their stories to Congress.

The two did not delve into specific proposals for Bush's aides, but Cornyn said he urged Fielding to release as much information related to the prosecutor firings as possible, warning that he wanted "no surprises" to emerge.

"I told him, 'Everything you can release, please release. We need to know what the facts are,"' Cornyn said.

Also Friday, the Justice Department said it had found additional e-mails, calendar pages and other documents about the dismissals and were working to send them to the House and Senate panels that oversee the Justice Department. It was unclear when those documents would be delivered.

Meanwhile, the flap over fired prosecutors is jeopardizing confirmation of President Bush's pick for the No. 3 spot at the Justice Department.

The nomination of Bill Mercer, currently the U.S. attorney for Montana, to become associate attorney general has been put on hold by Democrats. They want to question him about his role in the dismissal of several federal prosecutors late last year.

Mercer's name frequently pops up among more than 3,000 pages of e-mail exchanges among department and White House officials discussing the dismissals. The Senate Judiciary Committee has authorized a subpoena for Mercer as part of its investigation into the matter. His nomination is pending before that committee.

A committee spokeswoman said it will not consider the nomination until the investigation is finished; it is not clear how long that will take.

"In order for the committee to conduct a full and fair consideration of Mr. Mercer's nomination, it needs all the facts, including those that may arise as part of its investigation into the mass firings of the U.S. attorneys," said Tracy Schmaler, press aide for Leahy.

Mercer has served as acting associate attorney general in Washington since he was nominated in September. At the same time, he is the chief federal prosecutor in Montana, double-duty that has prompted criticism back home.

"I don't think it's appropriate for Mr. Mercer to hold both of these jobs," Democratic Sen. Jon Tester said in a statement Tuesday. "Montana needs a full-time U.S. attorney."

That same day, a spokesman for Montana's other senator, Democratic Sen. Max Baucus, said Baucus is hearing from lawyers and judges in the state who say Mercer's prosecutor's office is stretched too thin.

Mercer's top deputy in Montana, Kurt Alme, said last week that Mercer makes decisions from Washington and still handles a large share of his duties.

It is not unusual for U.S. attorneys to serve in temporary jobs at the Justice Department. But criticism of the practice has increased as scrutiny intensified over the prosecutor firings.

Rep. William Delahunt, D-Mass., criticized the department for allowing the U.S. attorney in Massachusetts, Michael J. Sullivan, to serve for the past six months as acting director of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

Bush this week nominated Sullivan for the job permanently, and his nomination is pending in the Senate Judiciary Committee. He is expected to continue in both roles pending Senate confirmation for the ATF job.

It is unclear when Sullivan's nomination will be brought up. Schmaler, the committee spokeswoman, said that the panel will consider nominations on an individual basis.

Delahunt, a member of the House Judiciary Committee, said it is unusual for Sullivan to perform two demanding full-time jobs. He blamed the department and a "lame duck" administration for Sullivan's double duty.

"Justice is in disarray right now," Delahunt said. "There are not many applicants for these particular jobs that have the appropriate background. ... When you combine all of these factors, you have creeping paralysis."

Chuck Rosenberg, U.S. attorney for the Eastern District of Virginia, also has two roles. He was named interim chief of staff at the Justice Department last week, after Sampson resigned in the uproar over the fired prosecutors. Rosenberg was confirmed by the Senate as U.S. attorney last June.

Department spokesman Brian Roehrkasse said it can be helpful for U.S. attorneys to gain a national perspective by working at the Washington headquarters.

Several other U.S. attorneys have done temporary duty at the department, he said, including Patrick Fitzgerald of the Northern Illinois district.

Fitzgerald led the investigation into the leak of CIA officer Valerie Plame's identity that brought the indictment and perjury and obstruction of justice convictions of I. Lewis "Scooter" Libby, Vice President Dick Cheney's former chief of staff.

This is not the first time Mercer has been criticized for working two jobs. In 2005, when Mercer served a temporary term at the Justice Department, Chief U.S. District Judge Donald Molloy in Montana chided Mercer for neglecting the state.