Clinton administration national security adviser Samuel R. "Sandy" Berger, under criminal investigation for removing copies of highly classified documents from the National Archives, severed his ties to John F. Kerry's campaign yesterday.

Berger, who has been the subject of an investigation since October, stepped down as Kerry's informal adviser on foreign policy and national security as the campaign moved quickly to stem the unfolding story's political damage.

A government official with knowledge of the probe said Berger removed from archives files all five or six drafts of a critique of the government's response to the millennium terrorism threat, which he said was classified "codeword," the government's highest level of document security.

A Kerry adviser said the expanding controversy convinced the campaign that Berger's departure was essential because of the serious distraction it posed for Kerry in the week before the Democratic Party nominates him for president.

Even as Berger acknowledged his actions, it remained unclear the degree to which they stemmed from carelessness or an intentional effort to hide and remove the documents, along with notes of the materials he was reviewing.

Berger's attorneys have acknowledged that he removed numerous classified memos, and apparently discarded some, as he reviewed materials on behalf of the Clinton administration for the independent commission investigating the Sept. 11, 2001, attacks. They said the removal of documents was inadvertent but that Berger was aware he was violating the law when he removed his handwritten notes without submitting them for review by National Archives staff.

Lanny Breuer, one of Berger's attorneys, said he was uncertain of the classification level of the various documents because he had not seen them but added that he believes that different versions of the critique bore different classification stamps. The documents that were removed were copies; the National Archives retained the originals.

Berger reviewed the millennium after-action memos during two visits to the archives last fall, his attorneys have said.

"I made an honest mistake which I deeply regret," Berger told reporters outside his office last night. "I dealt with this issue in October 2003 fully and completely. Everything that I have done all along in this process has been for the purpose of aiding and supporting the work of the 9/11 commission, and any suggestion to the contrary is simply absolutely wrong."

Republicans seized on the controversy. Sens. Saxby Chambliss (Ga.) and Gordon Smith (Ore.) speculated that Berger had provided the material to the Kerry campaign but offered no proof.

"Mr. Berger has a lot of explaining to do. He was given access to these documents to assist the 9/11 commission, not hide information from them," said House Speaker J. Dennis Hastert (R-Ill.). "The American people and the 9/11 families don't want coverups when it comes to the war on terror. They want the truth. And so does the U.S. House of Representatives."

Panel spokesman Alvin Felzenberg said yesterday that the panel is confident, based on records and other evidence, that it has been given copies of all the documents under investigation in Berger's case.

But Democrats said they were suspicious that the timing of the investigation's disclosure was intended to distract from the impact of the commission report, scheduled for release tomorrow.

Absent an exoneration from the FBI, the investigation appears to diminish, and possibly demolish, Berger's chances for a senior post in a Democratic administration should Kerry defeat President Bush, at least at the start of a new administration, according to some Democrats who have handled controversies relating to Cabinet nominations.

Kerry released a statement saying: "Sandy Berger is my friend, and he has tirelessly served this nation with honor and distinction. I respect his decision to step aside as an adviser to the campaign until this matter is resolved objectively and fairly."

Breuer said Berger removed the documents on Sept. 2 and Oct. 2, as he sifted through thousands of pages to determine which should be turned over to the commission. He said Berger inadvertently slipped drafts of the review -- each of which he said runs 12 to 15 pages -- into a leather portfolio he routinely carries.

Breuer said Berger also tucked handwritten notes he made from the classified documents into his pockets and removed them without allowing archives officials to review them, as required by law. He knew removing the handwritten notes was a violation, Breuer said, but he freely acknowledged having done so and turned the notes over when contacted by archives officials.

The National Archives has tight restrictions on the viewing of presidential materials, particularly classified documents, and normally does not allow researchers to bring portfolios or anything else with them. It is not clear why Berger was not held to those rules.

Archives officials discovered that some documents were missing after Berger's review of the files on Sept. 2, and again Oct. 2. The archives inspector general's office alerted Berger and former Clinton aide Bruce Lindsey, who is overseeing presidential documents. Berger returned two of the after-action drafts within days, according to his attorneys. Other drafts of the after-action document, they said, were apparently discarded. The archives inspector general began an investigation in October.

Breuer said he does not believe the Justice Department and the FBI are as interested in the handwritten notes as they are in the documents. "We have not been officially told how many documents they actually think are still missing," said David Fagen, another of Berger's attorneys. "We believe there are probably no more than two additional versions missing, but we don't know."

The FBI opened a criminal probe in January after receiving a referral from the archives inspector general. "This was not a routine investigation for this office for a number of factors," said Inspector General Paul Brachfeld, who declined to comment further.

The after-action memo was written by then-White House counterterrorism adviser Richard A. Clarke and contained 29 recommendations about improving homeland security, including beefing up protections at the nation's ports and borders. The review pointed out flaws in the nation's counterterrorism efforts.

A source knowledgeable about the contents of the review said that it is classified "codeword" because it contains information about sensitive intelligence operations. The memo also contains sensitive fruits of wiretaps, intelligence sources said.

During the months leading up to Jan. 1, 2000, the CIA and foreign intelligence services learned of terrorist plans to attack targets in the United States and abroad. Jordanian intelligence thwarted a plot to blow up the Radisson hotel in Amman, and an alert U.S. customs agent scuttled an al Qaeda plot to set off a bomb at Los Angeles International Airport when she apprehended terrorist Ahmed Ressam as he crossed into the United States from Canada.

Berger reviewed the documents on behalf of former president Bill Clinton and made recommendations to the Bush White House, which has final authority over which papers were given to the panel.

A Kerry campaign official, who declined to be identified in order to speak more freely about yesterday's internal discussions, said Berger had not informed the campaign about the investigation before news reports appeared Monday night. The official also said the campaign did not ask Berger to step down.

"The attitude here is he deserves the benefit of the doubt, especially given his record and experience," the official said.

In Democratic political circles yesterday, speculation was widespread on the personal damage to Berger -- who for two decades has been a voice of rising influence in the foreign policy establishment -- as well as on the possible explanations for how the incident could have happened. At best, Breuer acknowledged, the handling of the documents was extremely sloppy.

In the Clinton White House, Berger was known as a worrier -- forever alert to the kind of unexpected political controversy in which he is now embroiled. At the same time, he was known as someone who would constantly lose track of papers or appointments without subordinates to keep him organized and on schedule.

"For all those who know and love him, it's easy to see how this could happen," one former Clinton colleague said.

Staff writers John F. Harris, Dan Balz and Dan Eggen and researcher Meg Smith contributed to this report.

Sandy Berger has acknowledged removing papers from the Archives.