Attorney General John D. Ashcroft yesterday postponed the execution of Oklahoma City bomber Timothy McVeigh until June 11 and ordered a Justice Department investigation into how the FBI failed to give McVeigh's defense team thousands of pages of investigative files before his 1997 trial.

"Our system of justice requires basic fairness . . . essential to protecting the constitutional rights of every citizen and to sustaining public confidence in the administration of justice," Ashcroft said at a press briefing as he delayed what was to be the first federal execution in 38 years, originally set for Wednesday.

The attorney general bluntly conceded that "the FBI failed to comply fully" with an agreement to produce all the material for defense lawyers.

Rob Nigh, one of McVeigh's attorneys, said yesterday the defense may ask an appellate court in Denver for a longer stay of execution after it has an opportunity to review the 3,135 pages of documents turned over by the Justice Department on Thursday.

"A month is not enough time," Nigh said after emerging from a five-hour meeting with McVeigh at the federal penitentiary in Terre Haute, Ind. "You have to look at documents and analyze them in the context of the evidence in the case."

The material turned over to McVeigh's lawyers includes reports, correspondence, photographs, tapes and witness interviews taken soon after the bombing of the Alfred P. Murrah Federal Building on April 19, 1995. Although legal experts doubted it would provide evidence to overturn McVeigh's conviction, they said it might provide an opening to challenge his death sentence.

At the very least, the episode was an embarrassment to the FBI, exposing the bureau's sloppy handling of thousands of documents. News of the delay infuriated many relatives of the victims and survivors of the bombing, 300 of whom had planned to watch the execution on closed-circuit television in Oklahoma City. It also snarled plans for many in Terre Haute, which has spent months preparing for the arrival of thousands of protesters, reporters and onlookers.

Less than an hour after Ashcroft's announcement, President Bush commended the attorney general for making "the right decision." Bush said McVeigh was "lucky to be in America."

"This is a country that will bend over backwards to make sure that his constitutional rights are guaranteed, as opposed to rushing his fate," the president said.

Bush and Ashcroft stressed that the newly found documents would not cast any doubt on McVeigh's guilt -- or contradict McVeigh's own recent admission of guilt. The decorated Army veteran was convicted in June 1997 in Denver on all 11 counts of conspiracy and murder and was condemned to death. McVeigh, 33, dropped his appeals in December and let pass the deadline to request clemency from Bush.

"There is no doubt in my mind, or anyone's mind, about the guilt of Timothy McVeigh," Ashcroft stated.

The FBI's error stunned relatives of the victims.

"How can the FBI make this kind of a mistake?" said Dan McKinney, whose wife died in the blast. "I'm just ready to shut this chapter and I'm thinking it's never going to happen."

Kathleen Treanor, who lost her 4-year-old daughter in the explosion, said: "This is unforgivable, and someone needs to be held accountable. The ball is now back in McVeigh's court -- and it should never be there."

Ashcroft appeared at a loss yesterday to explain how 46 FBI field offices had failed to turn over the material from the bombing investigation that surfaced six days before the execution. Officials have said the missing records showed up when an archivist requested all reports connected to the McVeigh case be sent to Oklahoma City, a routine request when cases are about to be closed.

Sources said the material consists of approximately 700 actual items, most of which had not been seen by the defense. A number of items are "302 forms," which are official reports of interviews conducted by the FBI, and pertain to "John Doe No. 2," a suspect who was described by witnesses soon after the blast, but never found.

Asked if he had lost confidence in the FBI, Ashcroft hesitated and then avoided a specific response. "Obviously, this proceeding is an important proceeding," he said. "And we were making progress toward carrying this out without the kind of disruption that this causes. I regret that these steps which I have taken are necessary."

The materials also could affect the fate of McVeigh's convicted co-conspirator, Terry D. Nichols. Michael Tigar, Nichols's lawyer, said he planned to file an appeal on behalf of his client with the U.S. Supreme Court by last night's midnight deadline.

Nichols was convicted in federal court of conspiracy and involuntary manslaughter and sentenced to life in prison. He is awaiting trial on state charges in Oklahoma, where he could face the death penalty.

"We had a written agreement with the government. You bet your life I think it was deliberate," Tigar said when asked about the missing documents. "What I think happened is that [FBI Director] Louis Freeh declared the cases closed and then everyone started not complying with the agreement. . . . We've caught them in lie after lie after lie."

Justice Department attorney Sean Connelly, who has worked on the bombing case for a number of years in Denver, was first informed of the problems on Tuesday by the FBI. He informed McVeigh's lawyers by phone, and followed up with a letter and the documents on Thursday.

"We do not believe anything being produced [bears] on the federal convictions or sentences of Timothy McVeigh" or Nichols, he wrote.

A copy of Connelly's letter was sent to U.S. District Judge Richard Matsch, who presided over the separate trials of McVeigh and Nichols.

In the recently published "American Terrorist," McVeigh admitted for the first time that he blew up the federal building to avenge the 1992 government siege at Ruby Ridge, Idaho, where federal agents killed the wife and son of separatist Randy Weaver, and the 1993 federal raid on a religious cult near Waco, Tex., where 75 people died.

In an interview, Nigh said his client was "distressed" over the turn of events because McVeigh had "made the mental and psychological preparations to get ready to die. "He said his goodbyes to his family and friends. He is distressed that he has had to put these people through this process, only to have to put them through it again."

McVeigh has said he would rather die than spend the rest of his life in prison, but Nigh said McVeigh "is willing to take a fresh look and evaluate all the information."

Nigh said he does not have all the new documents but will review them immediately when he does.

Legal experts said yesterday that if McVeigh's attorneys find evidence that they believe would have helped defend him in 1997, they still face a federal appeals process that favor prosecutors. But the process also could delay McVeigh's execution for months.

The first pivotal legal proceeding likely would occur before a panel of the U.S. Circuit Court of Appeals in Denver, said David Bruck, a death penalty expert. He said McVeigh would need the panel's permission before presenting any new appeal to a lower federal court.

To obtain that permission, McVeigh would "have to show that [the documents] provide clear and convincing evidence of his innocence," said lawyer Richard Burr, who represented McVeigh during the penalty phase of his trial.

While federal law specifies the burdens McVeigh would have to meet to obtain a new trial, the law is much less clear on what he must prove in seeking a new sentencing hearing, lawyers said.

Legal experts noted the new information could call into question McVeigh's death sentence if it suggested the involvement of others, a fact that could have been considered by the jury as a mitigating factor.

In this case, the lawyers could point to the references to John Doe No. 2, which may have led a jury to believe that there was someone other than McVeigh who masterminded the crime.

A spokesman for the U.S. Bureau of Prisons said last night that the postponement of McVeigh's execution would not at this time affect the next scheduled federal execution -- that of Juan Raul Garza on June 19.

Joseph Hartzler, who led the McVeigh prosecution for the government, said yesterday there was every indication "there was not anyone else involved" in the crime.

The new material "may impact the date [of execution] but it won't impact the outcome," added Larry Mackey, who was on the prosecution team for both cases. "It's a tiny fraction of the kind of evidence that proved to be entirely immaterial to the question of guilt or innocence."

Staff writer Anne Hull, in Terre Haute, contributed to this report.

Bill and Amy Poffenbarger pause to reflect at Oklahoma City chapel across the street from memorial for victims of bombing.McVeigh attorney Rob Nigh said the defense may seek a longer stay of execution.