A batch of Hillary Clinton's private emails that were published Thursday by the State Department included at least one classified message that appeared to have been marked as "confidential" when it was written, which could contradict Clinton's longstanding argument that nothing she sent or received bore classification markings.

The record was among 275 emails that were upgraded to fully or partially classified in the latest trove of emails.

The email chain in question included a message from Harold Koh, a State Department legal adviser, that was listed as "confidential" both in the subject line and in the body of the email. "Confidential" is the lowest level of classification, followed by "secret." Most of Clinton's now-classified emails have been upgraded to the "confidential" level.

Alec Gerlach, a State Department spokesman, said Koh's email contained attorney-client discussions and was thus considered "confidential" only in the legal sense at the time.

"The phrase 'privileged and confidential' is often used by attorneys to indicate a likely attorney-client work product, and in no way indicates a national security classification, which is an entirely separate matter regulated by Executive Order 13526," Gerlach told the Washington Examiner.

However, a large portion of the Koh email was officially classified as "confidential" by the State Department before being posted online late Thursday afternoon.

Agency officials have frequently argued none of the emails were classified at the time they were written, and that all designations were made in light of information that has emerged since Clinton left office.

But the now-classified message from Feb. 2012 contained the word "confidential" in the subject and began with a clear disclaimer: "PRIVILEGED AND CONFIDENTIAL/ATTORNEY-CLIENT PRIVILEGE/DELIBERATIVE PROCESS/ATTORNEY WORK PRODUCT."

The email discussed ongoing negotiations with the Egyptian government over a U.S. non-governmental organization that was then facing legal trouble in the country.

The word "confidential" can denote classified information in an email, but does not always do so.

State Department officials maintained Thursday that none of the emails published were marked classified at the time they were sent, and said all 275 emails that were partially or fully classified in the latest batch were designated as such after the fact.