WASHINGTON (CNN) -- Staff from the Senate and House Judiciary Committees Wednesday are interviewing another Justice Department official as part of their probes into the firing of the eight U.S. Attorneys last year. They are questioning, behind closed doors, acting Associate Attorney General William Mercer.

Mercer was one of the DOJ officials involved in discussions at the department regarding the record of U.S. Attorney Carol Lam in San Diego on prosecuting immigration cases. Lam, one of the fired prosecutors, was repeatedly criticized in internal memos about the poor performance on that issue although she was not directly confronted about it.

Other Justice Department aides are expected to be on the hot seat later this week. Former Chief of Staff Kyle Sampson, who testified to the Senate Judiciary Committee March 29, is expected to be deposed Friday to ensure all remaining questions are answered prior to the testimony of Attorney General Alberto Gonzales next Tuesday before the Senate panel, a source close to Sampson confirms to CNN. Sampson and Gonzales have offered differing accounts of how involved the attorney general was in the process of firing the prosecutors.

Gonzales, meanwhile, is continuing his intense preparations for that appearance, which GOP political consultants and some members of Congress have said will be key to determining whether he can keep his job.