WASHINGTON, July 20 - Samuel R. Berger, the former national security adviser to President Bill Clinton, resigned abruptly Tuesday as a senior adviser to John Kerry's presidential campaign after the disclosure that he had improperly removed classified material on terrorism from a secure government reading room last year.
The decision came after Mr. Berger endured a day of furious criticism from Republican leaders, who accused him of breaching national security and possibly passing classified material to Mr. Kerry's campaign. Democrats, in turn, accused the Bush administration of leaking word of an F.B.I. investigation of Mr. Berger as a way of diverting attention from the release of the Sept. 11 commission's final report Thursday.
Mr. Berger told reporters Tuesday evening outside his Washington office: "Last year, when I was in the archives reviewing documents, I made an honest mistake. It's one that I deeply regret."
Associates of Mr. Berger said that although his mishandling of the classified material was inadvertent, he had decided late in the day to step down at least temporarily from the campaign because he did not want to detract from the Kerry effort.
"With that in mind, he has decided to step aside as an informal adviser to the Kerry campaign until this matter is resolved," said Lanny A. Breuer, a lawyer representing Mr. Berger in the investigation.
Mr. Berger's aides acknowledged that when he was preparing last year for testimony before the Sept. 11 commission, he removed from a secure reading room copies of a handful of classified documents related to a failed 1999 terrorist plot to bomb the Los Angeles airport. Republicans accused him on Tuesday of stashing the material in his clothing, but Mr. Breuer called that accusation "ridiculous" and politically inspired. He said the documents' removal was accidental.
The departure of Mr. Berger was at least a distraction for the Kerry campaign, which had hoped to gain political advantage from the Sept. 11 commission's anticipated criticisms of the Bush administration's handling of terrorism intelligence.
For months, Mr. Berger has consulted regularly with Mr. Kerry on the Iraq war, Middle East relations, terrorism and other foreign policy matters, helping to formulate speeches, prepare op-ed articles and brief reporters on the candidate's positions, campaign officials said.
"Sandy Berger is my friend, and he has tirelessly served this nation with honor and distinction," Mr. Kerry said Tuesday in a statement. "I respect his decision to step aside as an adviser to the campaign until this matter is resolved objectively and fairly."
Associates said he would probably try to rejoin the campaign after the Federal Bureau of Investigation had concluded an investigation that began in earnest in January after the National Archives discovered that classified material Mr. Berger had reviewed was missing.
But for Mr. Berger the damage may be difficult to overcome. Some Democrats suggested on Tuesday that the episode could severely hurt his chances of becoming secretary of state or taking another high-level cabinet position in a Kerry administration, jobs his name has been linked to.
Law enforcement officials said that the F.B.I. was continuing to investigate Mr. Berger's handling of the classified material and that the Justice Department had made no decisions about whether to seek criminal charges.
One crucial legal issue will be whether the evidence indicates that Mr. Berger's removal of the classified documents was inadvertent, as he and his lawyer assert. "That's clearly a question at the center of all this," said a law enforcement official who spoke about the investigation on condition of anonymity.
Though prosecutions for the mishandling of classified information are relatively rare, senior officials have become embroiled in such cases. In 2001, Mr. Clinton pardoned John M. Deutch, the former director of central intelligence, as he was negotiating a plea agreement with prosecutors over accusations that he had downloaded classified intelligence onto his unsecured computer.
Mr. Berger spent about 30 hours over three days in the summer and fall of 2003 reviewing classified material in a secure government reading room, his associates said.
Among those documents, officials said, were lengthy classified versions of an "after-action" report on the so-called millennium plots, which included a failed Qaeda effort to bomb Los Angeles International Airport in December 1999. The report on the plot was summarized in a staff report from the Sept. 11 commission earlier this year.
Mr. Breuer, the lawyer, said Mr. Berger inadvertently put three or four versions of the report on the plots in a leather portfolio he had with him. "He had lots of papers, and the memos got caught up in the portfolio," he said. "It was an accident."
Mr. Berger also put in his jacket and pants pockets handwritten notes that he had made during his review of the documents, Mr. Breuer said.
Officials at the National Archives realized late last year that several documents were missing and turned the matter over to the F.B.I., which later searched Mr. Berger's home and office, officials said. Mr. Breuer said that Mr. Berger had returned two of the documents, but that he had apparently discarded several others inadvertently.
Mr. Breuer said that the removal of even Mr. Berger's notes was a "technical violation," but he denied Republicans' assertions that Mr. Berger had removed the material intentionally to hide information that could be damaging to the Clinton administration.
J. Dennis Hastert, the House speaker, asked, "What information could be so embarrassing that a man with decades of experience in handling classified documents would risk being caught pilfering our nation's most sensitive secrets?"
And the House majority leader, Tom DeLay, said: "That is not sloppy. I think it is gravely, gravely serious what he did, if he did it, and it could be a national security crisis."
Mr. Breuer responded, "If there's a suggestion that he's shoving things down his pants, that is categorically false and ridiculous."
Democrats spent much of the day defending Mr. Berger as a man of integrity and asserting he had no reason to steal material already widely available to the Sept. 11 commission.
But late in the afternoon, Mr. Kerry's campaign announced that Mr. Berger was stepping down.
"It looked early in the day like this wouldn't be a big deal," said an associate of Mr. Berger who spoke on condition of anonymity, "but the Republicans made it partisan."
NY Times