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McVeigh attorneys, Nathan Chambers, left and Robert Nigh, speak with the media after May 16 meeting with Timothy McVeigh

DENVER, Colorado (CNN) -- Attorneys for Timothy McVeigh said Wednesday they still don't believe the government has provided all the documentary evidence the FBI gathered in the case and expect to file a motion Thursday seeking to postpone his June 11 execution.

But Attorney General John Ashcroft said he will "vigorously oppose" any attempt to further delay the execution of the man convicted of murder in the April 19, 1995 bombing of the Murrah Federal Building in Oklahoma City that killed 168 people.

Amnesty International, a longtime capital punishment foe, spotlighted the planned McVeigh execution in criticizing the U.S. government for sanctioning its first execution in 38 years.

IN-DEPTH • Execution of Timothy McVeigh • Oklahoma City Bombing    VIDEO In a press conference, U.S. Attorney General John Ashcroft discusses FBI handling of documents in the McVeigh bombing case (May 24) Part I | Part II (QuickTime, Real or Windows Media)    MESSAGE BOARD FBI: a troubled agency?    RESOURCES • Timeline: FBI under Louis Freeh • Previous FBI controversies   LEGAL RESOURCES Latest Legal News Law Library FindLaw Consumer Center Select a topic Bankruptcy Discrimination Divorce Estate Planning Landlord-Tenant Personal Injury Taxes  ALSO Paper: FBI document backs John Doe No. 2 theory

But Ashcroft, in a statement released by the Justice Department in Washington, said: "A jury determined that the death penalty is the appropriate punishment for McVeigh, and failure to carry out that sentence would deny justice for the victims of this crime and for the American people."

He said he acted five days before the original execution date of May 16 to order a one-month postponement of McVeigh's death by lethal injection to give his attorneys time to review 4,000 pages of documents the FBI said it inadvertently withheld from his trial lawyers.

"After an exhaustive review, it remains clear that none of the belatedly produced material raises any doubt about McVeigh's guilt," Ashcroft said.

McVeigh's attorneys Robert Nigh and Richard Burr plan to meet with their client Thursday morning at the Federal Penitentiary in Terre Haute, Indiana, for his permission to file for a stay of execution. The motion would then by filed before trial judge Richard Matsch in Denver by his other co-counsel, Nathan Chambers.

McVeigh believes the documents the FBI failed to provide his defense are "worthy of judicial review," Nigh said.

In addition, the attorney told CNN Wednesday, "We don't think that the government has turned over everything to us."

A Justice Department spokeswoman, Chris Watney, previously denied there are any additional documents that have not been provided.

Nigh said that McVeigh's attorneys cannot complete a review of more than 4,000 pages of newly provided documents before the June 11 execution date.

Jeralyn Merritt, a member of McVeigh's trial defense team, said she believes a motion for a stay would have "a good chance of prevailing because I think it is unreasonable to expect lawyers to not only evaluate but also investigate 4,000 pages of material in under a month.

"I also think new allegations that they purposely withheld exculpatory evidence in the FBI field offices and the lawyers believe that they may not to date have all the information would justify a stay," she said. "This is an execution. This is death ... What's the rush?"

William Schulz, executive director of Amnesty International, argued against executing McVeigh in discussing the group's 40th annual report on human rights abuses at a Washington news conference.

"If Timothy McVeigh or anyone else had been convicted of being a serial rapist, no civilized person would suggest that his punishment should be to be serially raped by officials of the government as punishment for his crime," Schulz said.

"Similarly, had he been a torturer, but not killed his victims, no civilized person would suggest that he should be tortured in return. And why not? Because we all know that the punishment that a government metes out is, in and of itself, a powerful statement of what is or is not acceptable behavior."