DENVER (AP) — Attorneys for Timothy McVeigh asked a judge Thursday to delay the Oklahoma City bomber's looming execution and accused the federal government of withholding evidence in a "fraud upon the court."

The request was submitted to U.S. District Judge Richard Matsch, who scheduled a hearing for late Thursday afternoon.

A few hours earlier, the attorneys met with McVeigh at a federal prison in Terre Haute, Ind. Attorney Robert Nigh said it wasn't easy for McVeigh to challenge his June 11 execution date.

"He was prepared to die," Nigh said.

The attorneys also want a hearing on their claim that the FBI is withholding information even now, three weeks after the Justice Department began turning over more than 4,000 pages of FBI documents that McVeigh's defense should have had at trial.

"There is a very long and old doctrine that the Supreme Court has articulated periodically because it doesn't happen very often, that when a fraud upon the court has been perpetrated by one of the parties to a legal proceeding, any judgment that the court makes is void," lawyer Richard Burr said.

"There are still critical documents about this investigation being withheld by the FBI," he said, suggesting the agency was keeping private files on people investigated in the case. "We must get to the bottom of this."

Attorney General John Ashcroft said none of the FBI documents raise doubt about McVeigh's guilt or establishes his innocence. He said the Justice Department would oppose any effort to overturn McVeigh's conviction or death sentence, or to force a new trial.

"Based on the overwhelming evidence and McVeigh's own repeated admissions, we know that he is responsible for this crime and we will continue to pursue justice by seeking to carry out the sentence that was determined by a jury," Ashcroft said in a statement.

McVeigh had faced lethal injection on May 16 for the 1995 bombing that killed 168 people and injured hundreds of others.

But the execution was postponed by Ashcroft after the Justice Department admitted it mistakenly kept boxes of documents from McVeigh's defense. The FBI said they were discovered by an archivist.

McVeigh admitted his guilt in a book released in April, but Nigh said such views were never aired by McVeigh during his trial or appeals.

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

"For many years, McVeigh has been deeply concerned about the overreaching of federal law enforcement authorities. When that overreach became apparent to him in his own case, it overrode other considerations."

Another attorney, Chris Tritico, bristled when asked whether the legal move was "offensive."

"I don't know how you could find it offensive that anybody would stand up for the principles as outlined in the Constitution," he said.

In Oklahoma City, Pat Ryan, who was U.S. attorney during the bombing, said nothing raised by McVeigh's attorneys Thursday 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.

"I think that's a crock," she said. "That is just McVeigh and his games. He is an admitted confessed, quoted killer so why should he receive a stay? He's lived six years plus longer than what my daughter did. So why should they stay it?"

Kathleen Treanor, whose 4-year-old daughter died in the bombing, said she wasn't surprised by McVeigh's decision.

"I've stopped trying to figure out what's going on in his head," she said. She said she's still reeling over the FBI's mistake.

"To be perfectly honest, I can't really say he got a fair trial at this point," she said. "It's back in the judge's hands now."