Within a two-month span, FBI field offices got conflicting advice from bureau archivists on what Oklahoma City bombing materials must be saved and which could be discarded, a government official said Monday.

The initial guidance went out in December, said the official, describing the situation on grounds of anonymity, four days after it was revealed the FBI withheld thousands of pages of evidence from lawyers for convicted bomber Timothy McVeigh.

By January, the source said, the archivists became concerned that some documents could be thrown out by mistake. So they contacted field offices again, this time with instructions to send all investigative materials to the Oklahoma City bureau, where the materials were being gathered and archived.

Archivists at that early juncture had discovered that a small percentage of the reports they were receiving had never been turned over to McVeigh's lawyers, the official said.

There was no indication that investigative materials that should have been turned over had been destroyed, but the possibility could not be ruled out, the official said.

The revelation was the latest twist to the FBI's mishandling of investigative materials in the Oklahoma City bombing case. The FBI failed to turn over to McVeigh's lawyers some 3,135 pages of investigative materials, including interview reports and physical evidence such as photographs, tapes and letters.

The FBI is trying to figure out what happened, said Mike Kortan, a spokesman.

"The FBI is tracking down the path of every document that the prosecution turned over to defense attorney's last week," said Kortan.

Asked if there are additional documents that have been discovered since the disclosure last week, Kortan said he was not aware of any but he added that the FBI was in the process of looking into the document mishap.

Attorney General John Ashcroft postponed McVeigh's execution, scheduled for Wednesday, until June 11 to give his attorneys time to review the documents, which were turned over last week. McVeigh's lawyers are poring through the documents; McVeigh is weighing whether the documents provide an opportunity to raise legal challenges to his conviction and execution.

A new CBS News polls showed that 69 percent of Americans agree with Ashcroft's decision to delay the execution.

Government prosecutors who worked on the McVeigh case never saw the documents either. They are now sifting through them as well.

Ashcroft said Justice Department attorneys have looked at the papers and don't think they contain anything that creates any doubt about McVeigh's guilt.

With McVeigh's execution set for May, the FBI at the end of last year was doing a routine archiving of all Oklahoma City bombing materials -- standard practice in wrapping up a case -- when the undisclosed documents were discovered.

Archivists sent field offices boilerplate instructions that established what types of documents should be archived and what could be discarded based on discussions with the National Archives and Records Administration, the official said.

The second memo went out when officials decided that decisions about what should be retained should be made by the archivists.

Law enforcement officials familiar with the matter have said that the newly disclosed documents are a small percentage of the millions generated during the investigation.

The FBI was moving to a new computer system when investigative documents were being filed electronically and some may have never been downloaded into set of master databases housing all Oklahoma City bombing records, they said. Many of the withheld documents are interview reports about a possible McVeigh accomplice who never materialized.

Meanwhile, Justice Department Inspector General Glenn Fine has launched an investigation into the belated disclosure, giving the probe priority status, said a Justice Department official.

Ashcroft requested the investigation on Friday.

The official declined to say how many investigators were working on the probe or when it would be complete. The inspector general's office, an internal watchdog division, has 350 employees, including 140 investigators and 140 auditors.

The IG has a full plate of FBI investigations. It has also launched a probe of the FBI's performance in preventing, detecting and investigating the alleged spying activities of Robert Hanssen, a veteran counterintelligence agent accused of spying for Moscow for 15 years.

Hanssen was arrested in February. Federal prosecutors and his lawyers are talking about a possible plea agreement. A preliminary hearing is scheduled for May 21, the deadline for the government to indict Hanssen.

The CBS News poll showed that more Americans view the FBI unfavorably than a year ago. Just 24 percent of those interviewed Friday and Saturday had a favorable opinion of the FBI, compared to 43 percent in April 2000.