A U.S. judge will likely delay Timothy McVeigh's execution, but probably not forever, say legal experts following the Oklahoma City bombing case.

Experts say it will be difficult for McVeigh's lawyers to prove the government intentionally withheld documents or that federal prosecutors knowingly presented false evidence.

"That's an enormous burden," said Scott Robinson, a defense attorney who monitored McVeigh's trial in Denver four years ago. "A simple mistake would never suffice."

After revealing their client would seek a stay of execution based on principle, McVeigh's attorneys on Thursday accused the government of "continuing to withhold numbers of documents" and perpetrating a "fraud on the court."

They asked U.S. District Judge Richard Matsch to delay McVeigh's execution, now set for June 11. Matsch scheduled a hearing for Wednesday.

Attorney General John Ashcroft, who said the Justice Department would oppose efforts to overturn the sentence or force a new trial, asserts the FBI had produced "every relevant document in its possession'' and none raises doubt about McVeigh's guilt.

McVeigh had asked Matsch in December to end his appeals, and the government later set his execution for May 16. But in early May, Justice Department officials admitted they had found thousands of documents the defense had never seen. Ashcroft order the execution delayed.

Since then, more than 4,400 documents and 16 hours of audio and visual material have been turned over to McVeigh's lawyers, the latest arriving Wednesday.

Robinson said Matsch will probably grant an execution delay to give McVeigh's attorneys time to review the material.

If McVeigh cannot prove fraud occurred, he will have to provide substantial new evidence to win either a new sentencing hearing or a new trial, said former Denver prosecutor Craig Silverman, who also watched the trial.

Ashcroft said the Justice Department would oppose efforts to overturn the sentence or force a new trial. "Based on the overwhelming evidence and McVeigh's own repeated admissions, we know that he is responsible for this crime," Ashcroft said.

In requesting the delay, McVeigh's attorneys accused the FBI of continuing to withhold evidence. They have said evidence may have been "intentionally destroyed" or not documented, and four former FBI agents have come forward to suggest information was withheld.

The most recent document was a report written by a former FBI agent who alleged this week on 60 Minutes II that the FBI ignored some of his reports that would have helped the defense. Former agent Rick Ojeda said a gag order prevented him from revealing any details.

Justice Department officials said the report was not related to the bombing case.

In a letter Wednesday to lawyers for McVeigh and co-conspirator Terry Nichols, federal prosecutor Sean Connelly said, "I am not aware of any FBI ... interview reports, by Ojeda or any other agent, that have not been turned over."

He said there was a report that Ojeda prepared "not as a part of the ... investigation, but rather at the specific request of the federal prosecutors and the FBI case agent" handling a different case. Connelly said the report would be provided to the defense.

"We categorically deny that there was any fraud on the court," Connelly said. "This is an effort to circumvent the legal requirement that requires McVeigh to show actual innocence in order to avoid his death sentence."

McVeigh, 33, admitted his guilt for the first time in a book released in April, and in a letter May 2 to The Houston Chronicle he repeated that he was responsible for the April 1995 bombing that killed 168 people at the federal building in Oklahoma City.

Defense attorney Rob Nigh said it was a difficult decision for McVeigh to seek the delay. "He was prepared to die," Nigh said.

Asked why McVeigh changed his mind, defense attorney Richard Burr said: "He right now thinks the most important thing in his life is to help bring integrity to the criminal justice system."

McVeigh's lawyers have suggested the FBI also kept private files on people investigated in the case. On Thursday, they asked Matsch to withhold details of their witnesses from prosecutors to prevent FBI agents from interfering in their investigation.

Matsch told Connelly to make sure the FBI does not try to pressure defense witnesses.

Connelly said the FBI cannot force people to cooperate with McVeigh's attorneys. "Some people contacted by McVeigh's investigators are chilled by that," he said.

In Oklahoma City, Pat Ryan, who was U.S. attorney during the bombing, said nothing raised by McVeigh's attorneys takes away from the evidence that McVeigh was responsible for the bombing.

"If death penalty crimes were ranked one to 100, this is 100," he said. "There has never been anything worse committed on American soil and Timothy McVeigh is going to get the death penalty at the end of the day."

Martha Ridley, whose daughter died in the bombing, said she didn't believe Nigh's statement that the decision to seek a stay wasn't meant to hurt the bombing victims.

"That is just McVeigh and his games," she said. "He's lived six years plus longer than what my daughter did."

The Associated Press contributed to this report