By MICHAEL BIESECKER and DEB RIECHMANN, Associated Press

WASHINGTON (AP) — The FBI is improperly restricting access to materials from its closed investigation into Hillary Clinton's use of a private email server, the chairman of the Senate Judiciary Committee said Wednesday.

In a letter sent to the Senate's security director, Iowa Sen. Chuck Grassley asked that unclassified portions of the FBI documents be provided to his staff. The move appears to be an end run around strict restrictions imposed by the FBI, which warned members of Congress not to leak documents from its investigation involving the Democratic presidential nominee.

Congressional aides told The Associated Press that the investigative materials demanded by House Republicans are being kept in a secure room on Capitol Hill typically reserved for the nation's most closely guarded secrets. The staffers spoke on condition of anonymity because they had not been authorized to speak publicly about the security precautions.

Documents containing classified information are included with those marked by the FBI as "Unclassified/For Official Use."

"As I have expressed to the FBI in the past, it is inappropriate to unnecessarily mingle classified and unclassified information," Grassley wrote Wednesday to Senate Security Director Michael DiSilvestro. "Accordingly, as you have done on similar occasions in the past, please provide the Judiciary Committee with a copy of the unclassified FBI documents from the production."

Grassley said it was "regrettable that the FBI has imposed the burden of this task on your office by improperly comingling so much unclassified material with classified material."

The FBI on Tuesday provided Congress portions of its file from the agency's yearlong investigation into whether then-Secretary of State Clinton and her top aides mishandled classified information that flowed through a private email server located in the basement of her New York home. Though he described Clinton's actions as "extremely careless," FBI Director James Comey said his agents found no evidence of criminal wrongdoing.

Republicans insist that Clinton lied to Congress about her handling of emails when she testified last October before a House panel investigating the deadly 2012 attacks in Benghazi, Libya. The GOP is pressing the Justice Department to open a new investigation into whether Clinton committed perjury and sought the FBI documents.

FBI case files are typically kept confidential after an investigation is closed without a recommendation for charges, and the Clinton documents were sent to Congress accompanied by written warnings not to leak the information.

"These materials are nonpublic and contain classified and other sensitive material," FBI Acting Assistant Director Jason Herring wrote. "For that reason, these materials may not be further disseminated or disclosed, in part or in full, without obtaining the FBI's concurrence."

The FBI documents are being kept in a Sensitive Compartmented Information Facility, known within the intelligence community as a SCIF.

Access to the guarded room is restricted to members of the oversight, judiciary and intelligence committees and their staffs. Those without sufficient security clearances can read only redacted versions of the files and are forbidden from making copies or taking notes.

Democrats, who have suggested Republicans are likely to leak portions of the FBI file selected to do political damage to Clinton, said they will comply with the FBI's request not to release any information without the agency's permission.

Rep. Adam Schiff of California, the top Democrat on the House Intelligence Committee, warned this week that providing the FBI's confidential notes to the Republicans will discourage witnesses from cooperating with future investigations.

"The history of the partisan Benghazi investigation made it clear that any information that can be leaked by the majority to the prejudice of Secretary Clinton, will be leaked," Schiff said.

Steven Aftergood, who directs the government secrecy project at the Federation of American Scientists, said the documents were provided to Congress as a matter of FBI discretion and the bureau is in a position to impose conditions on any further disclosures. He said if all the documents were unclassified, then it would be surprising — though not prohibited — for them to be stored in a SCIF.

"Since it includes both classified and unclassified material, it makes sense that the collection as a whole would be stored in a secure environment, such as a SCIF," he said. "The alternative would be to break up the materials into separate classified and unclassified parts and preserve them in separate locations, which might be awkward or inconvenient."

He said if members of Congress release some of the unclassified documents, that would not violate any law, but any such unauthorized disclosure would make it less likely that the FBI would voluntarily share information with Congress in the future.

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