A batch of Hillary Clinton's private emails set for release late Thursday evening will contain 45 newly-classified messages, State Department spokesman John Kirby said Thursday.

With the exception of one email, the documents will be classified as "confidential," the lowest level of classification. The remaining email will likely be upgraded to "secret," a higher level, as have a handful of others in previous batches of emails.

"In an effort to process and post as many documents as possible, what you'll see in today's release, the documents will not have fully completed data fields on the FOIA website, but it is searchable as they have been before," Kirby said, referring to the fact that the emails will not include subject lines or information about who sent or received the messages.

The State Department announced plans to publish 2,900 pages of Clinton's emails after falling far short of a court order requiring the agency to post 82 percent of the total number of records online by New Year's Eve.

Agency officials blamed the holiday schedule for both the shortfall and the lack of identifying markings on the emails. The absence of a subject line and other details makes the emails more difficult to search.

The email release Thursday will mark the ninth time the agency has published a significant trove of emails in response to a Freedom of Information Act lawsuit filed by Jason Leopold, a journalist for Vice News.

The high-profile FOIA case has occupied a third of the State Department's open records staff, according to a scathing FOIA report released by the inspector general Thursday.