Hillary Clinton emails contained 'top secret' material
Hillary Clinton's unsecured home server contained some closely guarded secrets, including material requiring one of the highest levels of classification, the US government has confirmed.
The revelation comes just three days before the Iowa presidential nominating caucuses in which Mrs Clinton is a candidate.
The State Department will release more emails from Mrs Clinton's time as US secretary of state later on Friday.
But The Associated Press has learned that seven email chains are being withheld in full for containing "top secret" material.
Department officials would not describe the substance of the emails or say if Mrs Clinton had sent any herself.
Spokesman John Kirby tells the AP that no judgment on past classification was made. But the department is looking into that, too.
Some 22 emails containing material demanding one of the highest levels of classification have been censored.
The 37 pages include messages recently described by a key intelligence official as concerning so-called "special access programmes" - a highly restricted sub-set of classified material that could point to confidential sources or clandestine programmes like drone strikes or government eavesdropping.
"The documents are being upgraded at the request of the intelligence community because they contain a category of top secret information," State Department spokesman John Kirby told the AP, describing the decision to withhold documents in full as "not unusual".
Mrs Clinton, the front-runner for the Democratic presidential nomination, has insisted she never sent or received information on her personal email account that was classified at the time.
No emails released so far were marked "classified" or "top secret", but reviewers had previously designated more than 1,000 messages at lower classification levels for public release.
For those that Mrs Clinton only read, and did not write or forward, she still would have been required to report classification slippages that she recognised. But without classification markings, that may have been difficult, especially if the information was in the public domain.
Mr Kirby said the State Department's focus as part of the Freedom of Information Act review of Mrs Clinton's emails was on "whether they need to be classified today". Questions about their past classification, he said, "are being, and will be, handled separately by the State Department".
Possible responses for classification infractions include counselling, warnings or other action, State Department officials said.
Mrs Clinton's main challenger for the Democratic nomination, Bernie Sanders, is neck and neck with her in the polls in Iowa and leads in New Hampshire. However, Mrs Clinton still holds a strong advantage in national polls.
The emails have been an issue for Mrs Clinton's campaign since it became known 10 months ago that she exclusively used a non-government account linked to a home server while in office. Mrs Clinton first called the decision a matter of convenience and then termed it a mistake, even if doing so was not expressly forbidden.
Both Mrs Clinton and the State Department have said her account was never hacked or compromised, which security experts assess as unlikely, and that the vast majority of her emails were preserved properly for archiving purposes because she corresponded mainly with government accounts. They have backtracked from the archiving claim, while the AP discovered several phishing attempts on her server connected to Russia.
The FBI is also looking into Mrs Clinton's email set-up but has said nothing about the nature of its probe. Independent experts say it is highly unlikely that she will be charged with wrongdoing, based on the limited details that have surfaced up to now and the lack of indications that she intended to break any laws.
The potential political costs that are probably of more immediate concern for Clinton. She has struggled in surveys measuring her perceived trustworthiness, and an active federal investigation could damage her national security reputation.