The probe of Berger, who served under former President Bill Clinton, escalated into an election-year firefight as Republicans accused Berger of stealing the documents for use by Kerry's campaign and Democrats questioned the probe's timing.

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

Berger said he had made a mistake and had dealt with the matter "fully and completely."

"Last year when I was in the archives reviewing documents I made an honest mistake. It's one that I deeply regret," he said in a statement to reporters.

"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," he said.

On Monday, Berger and one of his lawyers said he had inadvertently removed copies of a classified memo and his handwritten notes on the material as he reviewed it to determine what Clinton administration-era documents could be turned over to the commission looking into the Sept. 11, 2001, attacks.

The FBI, executing a warrant, searched Berger's office and home earlier this year.

Lanny Breuer, Berger's attorney, said he was told the government appreciated Berger's cooperation. "And then today, a couple of days before the 9/11 commission report comes out, the whole thing gets leaked," he said.

A Justice Department official refused to comment.

Republicans accused Berger of taking the documents so they could be used by the Kerry campaign at a news conference on port security, a charge Democrats said was ludicrous.

"Right after the documents were taken, John Kerry held a photo op and attacked the president on port security," said Rick Santorum, a Republican senator from Pennsylvania. "The documents that were taken may have been utilized for that press conference."

Democrats said the timing of the leak about the investigation and the Republican criticisms were meant to distract from Thursday's release of a report by the commission looking into the Sept. 11 attacks.

"I think the timing of all this is curious," Senate Democratic leader Tom Daschle of South Dakota said.

Kerry spokesman Phil Singer said Republicans were "playing politics with a criminal investigation" and also questioned the leak of the investigation and the Republican attacks.

"This appears to be a partisan attempt to divert attention away from the 9/11 commission report," Singer said.