FBI Director Louis Freeh acknowledged Wednesday a "serious error" in the bureau's failure to provide Timothy McVeigh's lawyers with evidence in the Oklahoma City bombing case. He said FBI headquarters had repeatedly asked, but failed, to get field offices to furnish the material.

Speaking publicly for the first time since disclosure that more than 3,000 pages of documents were withheld from McVeigh's lawyers at the time of his trial, Freeh said in prepared remarks that "I am not here to minimize our mistakes or to make excuses."

Even as he was explaining the problem, Freeh admitted that in addition to the 3,135 pages that have now been turned over, the FBI has located "a number of additional documents" as a result of a search ordered Friday.

The documents are being reviewed to see if they should be turned over, he said.

Freeh made the statement before a House Appropriations subcommittee, six days after the revelation and a day after testifying behind closed doors on the problem before the Senate Intelligence Committee.

"Regardless of how extraneous these documents are, if they were covered by the discovery agreement, they should have been located and released during discovery," Freeh said. "As director, I have taken responsibility. The buck does stop with me."

Nevertheless, he said, "the underlying case and his guilt remain unchallenged."

His testimony came on the same day that McVeigh was to have been executed for the bombing.

Freeh cited many requests that field offices send their material to its Oklahoma City field office — the operation that headquarters had assigned the job of compiling the records.

In 1995 and 1996, he said, field offices were told 11 times to send the documents.

When it appeared that not all materials had been sent, Freeh said he sent a priority teletype to all field offices in November 1996, directing all materials be sent promptly.

"As we now know, there were still many offices that had failed to comply fully or precisely with the instructions given," Freeh said.

"As a consequence, the items now at issues were apparently never turned over to the prosecutors during the discovery period."

Subcommittee Chairman Frank Wolf, R-Va., asked Freeh why the FBI did not do a check by hand for relevant documents — in addition to an electronic search.

"That is one of the questions we'll have to answer," Freeh said.

Asked by Wolf if his own directives to FBI field offices on the matter might have been "fuzzy," Freeh said no, they were "absolutely clear ... that everything and anything was to be retrieved and sent to Oklahoma City."

Rep. David Obey of Wisconsin, the senior Democrat on the House Appropriations Committee, told Freeh at the onset of the hearing he believes the FBI has a "litany of troubles."

"I think we have today something close to a failed agency," Obey said. "There is such a pervasive list of problems through the years," he added, citing the Ruby Ridge shootout in addition to the lost McVeigh papers.

Freeh said he regrets the pain that the FBI mistake involving McVeigh has caused the victims and family members who lost loved ones in the blast. McVeigh was convicted of the April 19, 1995 bombing of the Alfred P. Murrah Federal Building, in which 168 people died, including 19 children.

The missing documents were only found when FBI archivists started collecting material for storage and discovered that materials in the field offices had never been turned over to prosecutors. Instead of just storing or hiding the documents, the archivists turned the material over to their superiors, who turned it over to the lawyers, Freeh said.

That was "not the easiest thing to do, but the right thing to do," Freeh said.

The FBI has come under withering criticism in recent days, and Attorney General John Ashcroft last week postponed until June 11 McVeigh's execution for the bombing.

Freeh said most FBI offices either failed to locate the documents, misinterpreted their instructions to send the documents, or sent the documents, only to have them unaccounted for on the other end.

"Any of these cases is unacceptable," said Freeh, who had announced earlier that he was retiring after eight years at the helm rather than serving the full 10-year-term, which would have ended in 2003.

Freeh outlined a series of problems, mistakes and mishaps that contributed to the bureau's failure to turn over all documents.

At the time that all investigative documents were supposed to have been sent to the Oklahoma City office for uploading to a computer system, Freeh said, the FBI was converting to a new computer system.

Freeh said that during the first six months of the investigation, the bombing command post in Oklahoma City had trouble ensuring that all field offices coordinated their investigative materials with records maintained in Oklahoma City.

Sen. Richard Shelby, R-Ala., chairman of the Senate Intelligence Committee, emerged from Tuesday's session with sharp criticism of the FBI.

"I don't think you can blindly have confidence in anything," Shelby told reporters. "It does cause us all to be concerned about some of the goings on, lack of efficiency, lack of judgment perhaps, at the FBI."

"It's something that should not have happened, and it shows, probably, a lack of diligence somewhere in the FBI," Shelby said. The bureau, he said, has had "too many failures, too many blunders" of late.

The Associated Press contributed to this report.