The FBI has informed the federal judge that sentenced Oklahoma City bomber Timothy McVeigh and McVeigh's attorneys that the bureau withheld evidence from his trial, Fox News has learned.

The FBI turned over thousands of documents from the Oklahoma City bombing investigation to McVeigh's defense team Thursday. An attorney said the material may form the basis for a stay of execution request.

"We're considering all our options," McVeigh lawyer Nathan Chambers said after the documents, used in a separate trial involving McVeigh's co-defendant Terry Nichols, were delivered to his office in Denver.

"We're considering all our options," McVeigh lawyer Nathan Chambers said after the documents, which the FBI said were mistakenly withheld, were delivered to his office in Denver.

Chambers said he had spoken to McVeigh about the documents but declined to reveal what he said. "Mr. McVeigh is going to think about it and decide how he wants to proceed," he said.

The disclosure was a jarring development, coming as federal law enforcement officials make final preparations for McVeigh's lethal injection at a federal prison facility in Terre Haute, Ind., next Wednesday.

The Justice Department said "a number of FBI documents" should have been provided to McVeigh's attorneys during the discovery phase of his 1997 trial in Denver federal court.

"While the department is confident the documents do not in any way create any reasonable doubt about McVeigh's guilt and do not contradict his repeated confessions of guilt, the department is concerned that McVeigh's attorneys were not able to review them at the appropriate time," the agency said in a statement.

The agency said it has asked defense attorneys for notification if they believe the documents throw McVeigh's guilt into question.

An assistant in the attorneys' office told Fox News that they received a call from the government today explaining the situation and they are currently determining the impact.

James Manspeaker, the U.S. District Court clerk for the 10th Circuit, told Fox News that Judge Richard Matsch, presiding judge in the Timothy McVeigh and Terry Nichols trials, has received a copy of a letter advising that the U.S. Attorney has sent a letter to legal counsel for McVeigh and Nichols advising that they have come across some additional documents. 

Nothing is pending before the court, no motions — so far the court is not involved.

Pat Ryan, the lead prosecutor in the case and the man that got McVeigh the death penalty told Fox News that he's offended that the government withheld information from the courts.

Ryan said he wants to know why the government chose to reveal this now — just six days before the execution. He said he was sure his team turned over everything it had to the courts, but he believes Judge Matsch should grant a stay because information was mistakenly withheld.

He said that in his opinion, this turn of events is going to be unsettling to all of the bombing victims.

The mistake was discovered as the bureau was gathering up documents for archiving, said an official, speaking on condition of anonymity.

The disclosure was a jarring development, coming as federal law enforcement officials were making final preparations for McVeigh's execution by lethal injection at a federal prison facility in Terre Haute, Ind., next Wednesday.

McVeigh has shown no inclination of any plan to seek a stay of execution for the 1995 bombing of the Alfred R. Murrah Federal Building in Oklahoma City, which killed 168 people.

Other law enforcement officials said the FBI informed the trial court of the oversight Tuesday.

The documents were discovered recently by the FBI and included evidence such as interview reports from the early investigation, one official said. When asked the quantity of the withheld evidence, that official answered, "Boxes." CBS News, which first reported the foul-up, said that about 200 documents had been withheld.

A source said that an initial FBI review indicated the withholding was accidental and not an attempt to thwart helping McVeigh's defense.

Chambers declined to say whether the documents would be favorable to an appeal by McVeigh, who has admitted to the bombing and has ordered his attorneys not to appeal his conviction or death sentence.

"If it becomes an issue, the courts will deal with it," he said.

Chambers said he had talked to McVeigh but refused to disclose what he said.

Fox News' Alicia Acuna, Carol McKinley and The Associated Press contributed to this report