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WASHINGTON (CNN) -- Death penalty foes say they have been given a boost by the revelation that the FBI withheld documents from Timothy McVeigh's defense team.

The FBI admission "does raise the prospect that in some cases, we are risking innocent lives," said Richard Dieter, executive director of the Death Penalty Information Center.

FBI officials said more than 3,000 pages of notes, transcripts and investigative files were inadvertently omitted from the material it provided to McVeigh's legal team prior to the trial.

VIDEO CNN's Kelli Arena says Louis Freeh met with senators who are troubled by recent problems at the nation's top law enforcement agency (May 15) Play video (QuickTime, Real or Windows Media) CNN's Kelli Arena reports on an outdated computer system that the FBI blames for the missing McVeigh evidence (May 12) Play video (QuickTime, Real or Windows Media) CNN's Wolf Blitzer reports on what the McVeigh execution delay may mean for McVeigh and Nichols (May 11) Play video (QuickTime, Real or Windows Media) McVeigh lawyer Robert Nigh says his client will evaluate new information to decide whether he now wants fo fight execuition Play video (QuickTime, Real or Windows Media)  DOCUMENTS What the FBI withheld: • About 3,100 pages of documents and pieces of evidence including "Form 302" documents that give the essence of interviews that FBI agents conducted following the Oklahoma City bombing. MORE DETAILS FBI documents from special agent in charge (FindLaw) (PDF format) Documents in PDF format require Adobe® Acrobat® Reader™ for viewing.  ALSO • Ashcroft: No more delays for McVeigh • Keating: 'How can there be anything wrong?' • Previous FBI controversies • Newly revealed FBI documents prompt Nichols' appeal More on the McVeigh execution  MESSAGE BOARDS • McVeigh execution • Death penalty

McVeigh's execution -- postponed until June 11 -- has not drawn the kind of widespread protests generated by previous cases. "It's not that people are going to start rallying and think that McVeigh shouldn't be executed," Dieter conceded.

Still, several anti-death penalty groups, such as Amnesty International, have opposed his sentence. Some vigils had been planned outside the federal prison in Terre Haute, Indiana, where the execution was originally scheduled to take place Wednesday.

McVeigh's case, however, never touched on racial bias or mental retardation, points that have become rallying cries for critics of the death penalty.

"This case clearly is being noticed, but it doesn't raise those issues," Dieter said.

It is, however, raising the issue of whether a mistake at the trial level -- the failure of the FBI to disclose all of its documents -- calls into question the verdict or the validity of McVeigh's death sentence.

Sister Helen Prejean, a nun whose opposition to the death penalty was featured in the movie "Dead Man Walking," said the latest developments in the McVeigh case underscore her belief that a sentence that leaves no room for doubt has no place in a criminal justice system where mistakes occur.

"Innocent people are going in there with the guilty," she said, adding, however, that is not the case with McVeigh.

Death penalty opponents stress that the U.S. criminal justice system is not infallible. They point to several cases in recent years when men convicted of certain crimes were released from prison because new DNA testing cleared them.

"One can only begin to imagine the mistakes made in trials that lack the level of public scrutiny and expertise of this one," said a statement from The Justice Project, a group opposed to the death penalty.

But supporters of the death penalty said the McVeigh case only highlights the safeguards in the system. They point out that Attorney General John Ashcroft postponed the execution when the new documents came to light.

"I believe the death penalty is applied fairly," said Dianne Clements, with Justice for All, a Texas-based criminal justice reform group. "I know that states are always tweaking their statutes in order to give due process even more fairly to those who are convicted and accused of the most heinous crimes."

An estimated 3,270 inmates are on death row in the United States, according to the Death Penalty Information Center.