President Obama emailed Hillary Clinton using a pseudonym while she served as his secretary of state, according to FBI documents released Friday.

The president’s previously unreported use of a pen name is referenced in notes from federal investigators’ April 5, 2016 interview with Huma Abedin, one of Mrs. Clinton’s closest aides, contained within 189 pages of records released late Friday afternoon by the FBI concerning its review of the Democratic presidential nominee’s use of a private email server while in office.

During that interview, investigators showed the aide an email exchange dated June 28, 2012 with the subject “Re: Congratulations!”

“Abedin did not recognize the name of the sender. Once informed that the sender’s name is believed to be a pseudonym used by the president, Abedin exclaimed ‘How is this not classified?’” according to the FBI’s summary of the interview.

“Abedin then expressed her amazement at the president’s use of a pseudonym and asked if she could have a copy of the email.”

The FBI’s revelation quickly spurred questions about the president’s past claims concerning his knowledge of Mrs. Clinton’s private email server. Mrs. Clinton’s non-governmental email address was first revealed in 2013 when a Romanian computer hacker breached the AOL account of Sidney Blumenthal, a longtime Clinton confidant, and subsequently leaked messages to the media that were sent to an account operated by Mrs. Clinton’s outside of the .gov realm.

The New York Times reported in March 2015 that congressional investigators determined Mrs. Clinton had used her private email server exclusively while in office, and the president said days later then he became aware of her set-up at the “same time everybody else learned it through news reports.”

Thousands of emails from Mrs. Clinton’s personal account have since been released by the State Department, but the president’s attorneys have prevented the disclosure of any correspondence involving Mr. Obama by citing “presidential communications privilege.” As a result, neither Mr. Obama’s pseudonym nor the contents of his email with Mrs. Clinton, including the one shown to Ms. Abedin, have been made publicly available.

The notes released Friday are the most recent items to be made available to the public after the FBI decided in July not to pursue charges against Mrs. Clinton concerning her use of a private email account to conduct official business.

“These materials further demonstrate why the Justice Department believed there was no basis to move forward with this case,” Brian Fallon, a spokesman for Mrs. Clinton, told the New York Post following Friday’s release.

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