Senators investigating the firings of federal prosecutors said Thursday they must have details of Attorney General Alberto Gonzales' role before he can testify.

The request by the Senate Judiciary Committee chairman, Sen. Patrick Leahy, came as a separate Senate panel shelved its own meeting next week with Gonzales because the firings have overshadowed all other issues connected to him.

"It would be very difficult in this environment to give the department's budget request the attention it deserves until the Senate has examined the department's leadership failures," said Sen. Barbara Mikulski, D-Md. She heads the Senate Appropriations subcommittee that oversees Justice Department spending and had planned to hear from Gonzales on April 12.

The delay further frustrates the White House's push for Gonzales to give lawmakers his side of the story as Democrats and Republicans alike call for his resignation over the botched U.S. attorney firings.

Gonzales has been forced to clarify his role in the firings after first saying, on March 13, he "was not involved in seeing any memos, was not involved in any discussions about what was going on." That statement was later contradicted by documents and testimony by Kyle Sampson, formerly the attorney general's top aide.

Sampson told Leahy's committee that Gonzales was briefed regularly about the firings and "this process of asking certain U.S. attorneys to resign."

Gonzales says he was not involved in selecting which prosecutors to dismiss and largely relied on Sampson to orchestrate the firings.

White House spokesman Tony Fratto said the delay was "disappointing."

"The attorney general has said that he would testify in hearings regarding Department of Justice appropriations, U.S. attorneys, or other issues of interest to the Senate whenever they could be scheduled," Fratto said.

Leahy, in a letter dated Wednesday and released Thursday, noted that his committee set its hearing for April 17 — a date initially requested by Gonzales.

He asked Gonzales to produce "a full and complete account of the development of the plan to replace United States attorneys and all the specifics of your role in connection with that matter." Leahy, D-Vt., asked for the information at least two days before the hearing, but added that "nothing prevents you from providing" it earlier.

Leahy criticized Gonzales for failing to answer about 200 written questions following his January appearance before the committee. At the time, he was asked about the firings, as well as a secret court's oversight of spying on suspected terrorists and FBI leaks in corruption investigations.

"You would not tolerate this kind of response time in a Justice Department investigation where months go by without answers and when those answers are finally provided they are outdated or superseded by events," Leahy wrote. "That is not conducive to effective oversight."

The department had no immediate comment Thursday.

Pennsylvania Sen. Arlen Specter, the top Republican on the Judiciary Committee, said Gonzales "will have to answer some important questions" at the hearing.

"He's going to have to explain what appears to be inconsistent statements which he made when he said he was not involved in discussions when e-mails showed that he was at meetings," Specter said in Pittsburgh.

The senator said the attorney general also will have to address the FBI's improper and, in some cases, illegal use of national security letters to get personal client data from telephone and Internet companies.

"There was an important issue of misuse of national security letters," Specter said. "Those are the issues he will have to face."

Meanwhile, the former U.S. attorney in New Mexico, David Iglesias, said he is talking with the government's independent counsel about whether department officials violated federal law when they included him among the fired prosecutors.

The Office of Special Counsel is looking into whether Iglesias' firing may have violated a law that protects military reservists from discrimination. It is also is examining possible violations of laws designed to protect whistle-blowers and prohibit political activity by government employees, Iglesias said in an interview this week.

Iglesias said he has authorized an investigation. "It's too early to tell whether will result in a legal claim," he said.