Former US national security adviser Samuel "Sandy" Berger pleaded guilty to taking classified documents from the National Archives while preparing for an investigation into the September 11 attacks.
Mr Berger, who served under former President Bill Clinton, pleaded guilty to the misdemeanor in the US District Court and will be sentenced on July 8.
Under a plea deal, he will not serve jail time but must pay a $10,000 fine.
Mr Berger, who also agreed to surrender his US government security clearance for three years, was forced to step down as an adviser to Democratic presidential candidate John Kerry last summer after reports emerged of the inquiry into the incident.
He admitted to taking copies of five documents from the National Archives in late 2003 while preparing for testimony before the commission investigating the September 11, 2001 hijacked-airline attacks.
"I exercised very poor judgment in the course of reviewing the files," Mr Berger told reporters outside the courthouse after pleading guilty.
"I deeply regret it. It was mistaken and it was wrong."
"My motivation was to help prepare myself and others," he said.
The plea agreement is subject to approval by Magistrate Judge Deborah Robinson.
According to the facts admitted by Berger, who knew he was not authorized to remove classified documents from the Archives, he concealed and removed one copy of a document on September 2, 2003 and copies of four others on October. 2, 2003.
He put the documents into his suit pockets to get them out of the Archives.
He took them to his office and used scissors to destroy three of the copies.
Mr Berger also admitted to smuggling out handwritten notes from the Archives.
The notes have since been returned to the government.
He would not answer questions as to why he decided to destroy three of the documents.