President Barack Obama used a pseudonym when communicating with then-Secretary of State Hillary Clinton by email, and at least one of those emails ended up on Clinton’s private email server, new FBI documents reveal.

The FBI dumped a large batch of documents Friday concerning its investigation into Clinton’s private email server. The heavily-redacted documents include numerous notes the Bureau made of interviews with Clinton aides and Department of State officials concerning her private email server.

One of those interviews is with Clinton aide Huma Abedin. During the interview, the FBI presented Abedin with an email exchange between Clinton and a person she did not recognize. Eventually, the FBI revealed the unknown person’s name was believed to be a pseudonym used by Obama. The revelation provoked consternation from Abedin.

“How is this not classified?” She said, before asking for a copy of the email herself.

The FBI documents do not reveal what Obama’s pseudonym was, or what was in the text of the email. The subject, “Re: Congratulations,” combined with the date (June 28, 2012) suggests it may have been about the Supreme Court’s ruling upholding the constitutionality of Obamacare.

The email was never previously released, with lawyers citing “presidential communications privilege” to avoid turning them over under the Freedom of Information Act.

The revelation raises questions about Obama’s claim that he knew nothing about Clinton’s private email server. When news of the server started to emerge in 2015, Obama said he learned about it by watching the news just like everybody else did. Obama has never denied knowing Clinton’s email address, though since it used the domain “clintonemail.com,” that would be a giveaway to an observant person that Hillary was using a non-government email.

It’s also unclear what reason Obama had for using a pseudonym, though he wasn’t the only person to use one. Notably, Clinton’s daughter Chelsea Clinton frequently went by the alias “Diane Reynolds.”

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