WASHINGTON — 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.