Timothy McVeigh's lawyers went to U.S. District Court in Denver on Thursday to ask a judge to delay the Oklahoma City bomber's scheduled June 11 execution.

U.S. District Judge Richard Matsch scheduled a hearing for Wednesday and ordered prosecutors to respond by Monday evening. McVeigh is scheduled to die June 11. 

The lawyers asked for a hearing to investigate what they said was "fraud upon the court" by federal investigators.

They submitted their sealed request to U.S. District Judge Richard Matsch, who presided over McVeigh's 1997 trial. Matsch's clerk, James Manspeaker, declined to say how quickly the judge would act.

Essentially, McVeigh is charging that the Justice Department cheated in his prosecution by deliberately withholding evidence that might have persuaded a jury to find him not guilty of murder. He and his lawyers believe that is sufficient reason to overturn his conviction.

"There's a very long and old doctrine that the Supreme Court has articulated periodically because it doesn't happen very often: When a fraud upon the court has been perpetrated by one of the parties in a legal proceeding, then any judgment that the court makes is void," lawyer Richard Burr said earlier after meeting with McVeigh at the federal penitentiary in Terre Haute, Ind.

"There are still critical documents about this investigation being withheld by the FBI," Burr said. "We must get to the bottom of this. This proceeding is intended to do that."

Outside the courthouse Thursday evening, prosecutor Sean Connelly said, "We categorically deny there was any fraud on the courts." 

Defense lawyers suggested the FBI was keeping private files on people investigated in the case. During the hearing, 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. 

"There are a lot of accusations about that agency and the conduct of its people," Matsch said. "I want to be confident this word gets out." 

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. 

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. 

Attorney General John Ashcroft said the FBI documents neither raise doubt about McVeigh's guilt nor establish his innocence. He said the Justice Department would oppose any effort to overturn McVeigh's conviction, death 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 and we will continue to pursue justice by seeking to carry out the sentence that was determined by a jury," said Ashcroft, who was traveling in the Netherlands.

Lead attorney Robert Nigh said it wasn't easy for McVeigh to make the decision.

"He was ready to die," Nigh said. "He preferred death to life in prison with no release.

"It is upon principle that he has decided to seek court relief. The Department of Justice and the FBI will not otherwise be held to account unless he takes this action.

"He understands and recognized the impact that his decision will have upon his family, the victims, the community of Terre Haute, and others.

"His decision in no way stems from the desire to cause these people any additional pain or trauma. It is his belief that this action is necessary in order to promote integrity in the criminal system."

Earlier in the day, the lawyers met with the convicted mass murderer in the federal penitentiary where he is scheduled to be put to death.

McVeigh was sentenced to receive a lethal injection for killing 168 people in the 1995 bombing of the Alfred P. Murrah Federal Building. He was originally scheduled to die on May 16 in what would have been the first federal execution in 38 years. That was postponed when the Justice Department turned over thousands of FBI documents it admitted should have been given to McVeigh's lawyers during his trial.

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 is guilty.

"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."

The Associated Press contributed to this report