WASHINGTON (AP) — For an agency still reeling from the discovery of an alleged spy in its ranks, the last thing the FBI needed was the disclosure that it withheld evidence from lawyers representing the man convicted of the worst act of domestic terrorism.

Timothy McVeigh was scheduled to be executed by lethal injection on Wednesday. Now his lawyers are weighing whether to seek a stay of the execution, which would have been the first federal death penalty sentence carried out since 1963.

The mishap comes a little more than a week after FBI Director Louis Freeh said he plans to retire in June — two years short of his 10-year term. Law enforcement officials familiar with the case said there was no connection between Freeh's decision to retire and the problem with the McVeigh documents.

The revelation shook the law enforcement establishment — and people waiting to see closure more than six years after a bomb destroyed the Alfred P. Murrah Federal Building in Oklahoma City, killing 168, including many women and children.

"I'm appalled," said Kathleen Treanor, who lost her 4-year-old daughter and her in-laws in the bombing. "The FBI knew from the very beginning that this was a huge case. How could they have possibly made a mistake this huge?"

The documents mishap also follows the arrest in February of Robert Philip Hanssen, a 20-year veteran agent accused of selling national secrets to Moscow.

Hanssen, a counterintelligence agent with access to highly sensitive information, carried on his alleged spying activities for 15 years without being detected by his bosses. Investigations are underway to figure out why.

Other controversies, from a crime-lab scandal in the 1990s to the botched investigation last year of former Los Alamos scientist Wen Ho Lee, have dogged the FBI in recent years.

The revelation that some 3,135 investigation materials — including interview reports and physical evidence such as photographs, letters and tapes — were inadvertently withheld from McVeigh's attorneys is another embarrassment for the FBI.

Law enforcement officials familiar with the documents mishap, speaking only on condition of anonymity, said the mistake resulted from an antiquated records system. The FBI was in the routine process of gathering all documents from the Oklahoma City bombing investigation — numbering more than 1 million — from its bureaus when officials discovered that some pages had never been shared with defense lawyers.

"One thing that's overlooked here is that there were thousands and thousands of these statements that have to be stored and catalogued," said Andrew Cohen, a legal analyst who has followed the case. "Certainly you don't want to encourage the government to lose this sort of thing, but in some ways it's a bit understandable."

As soon as the mistake was discovered, the bureau acted quickly to turn the documents over, the sources said. The Justice Department received the documents Wednesday and sent McVeigh's attorneys copies of everything.

The department says none of the documents create any doubt about McVeigh's conviction or sentence. McVeigh's lawyers could still ask for stay of the execution so they can examine the materials.

"I think the FBI has given McVeigh the chance to delay his own execution," said Cohen.

Paul Heath, who was injured in the bombing, said he was taking a wait-and-see approach to the news.

"I'm convinced it wouldn't make any difference to Mr. McVeigh," said Heath. "It does not upset me."

Last year the FBI was stung by the case of Wen Ho Lee, a former Los Alamos scientist indicted on 59 criminal counts of mishandling nuclear weapons secrets. He spent nine months in solitary confinement in a New Mexico jail. All but one count was eventually dropped.

The FBI also suffered through an embarrassing investigation by the parent Justice Department of its world-renowned crime lab in the mid-1990s.

Spurred by allegations from Frederic Whitehurst, an FBI lab chemist, Justice Inspector General Michael Bromwich investigated the facility for 18 months. He subsequently blasted the FBI facility for flawed scientific work and inaccurate, pro-prosecution testimony in major cases including the Oklahoma City bombing.

The catastrophe at the Branch Davidian compound in Waco, Texas, that killed 80 people and a shoot-out with white separatists in Ruby Ridge, Idaho, also have dogged the FBI.

McVeigh has said he carried out the Oklahoma City bombing to avenge the deaths at Waco and Ruby Ridge.