The AP is reporting that a group of emails meant to be released today as part of the investigation into Hillary Clinton's personal email server are being withheld because they contain "top secret" information.
It has not been reported whether any of the information was classified at the time of transmission.
State Department spokesman John Kirby clarified Friday, "The documents are being upgraded at the request of the Intelligence Community. These documents were not marked classified at the time they were sent."
Kirby told the AP that the decision to withhold documents in full as "not unusual."
The Associated Press learned seven email chains are being withheld in full from the Friday release because they contain information deemed to be "top secret." The 37 pages include messages recently described by a key intelligence official as concerning so-called "special access programs" — a highly restricted subset of classified material that could point to confidential sources or clandestine programs like drone strikes or government eavesdropping.
Even if Clinton only read, and didn't write or forward the secret messages, she still would have been required to report classification slippages that she recognized. But without classification markings, that may have been difficult, especially if the information was in the public domain.
Department officials wouldn't describe the substance of the emails, or say if Clinton sent any herself.
"We firmly oppose the complete blocking of the release of these emails," Clinton campaign spokesman Brain Fallon said in a statement. "Since first providing her emails to the State Department more than one year ago, Hillary Clinton has urged that they be made available to the public. We feel no differently today."
Fallon accused the "loudest and leakiest participants" in a process of bureaucratic infighting for withholding the exchanges. The documents, he said, originated in the State Department's unclassified system long before they ever reached Clinton, and "in at least one case, the emails appear to involve information from a published news article."
"This appears to be over-classification run amok," Fallon said.