The Obama administration has confirmed for the first time that Hillary Clinton’s home server contained closely-guarded government secrets, censoring 22 emails with material requiring one of the highest levels of classification.
The revelation comes three days before the Democrats’ presidential front-runner competes in the Iowa caucuses.
State department officials also said the agency’s Diplomatic Security and Intelligence and Research bureaux were investigating if any of the information was classified at the time of transmission, going to the heart of Mrs Clinton’s defence of her email practices.
The department published its latest batch of emails from her time as US secretary of state.
Republicans have been trying to slow the momentum of Mrs Clinton’s campaign by playing up her email issues, saying she made her own rules when it came to national security.
But the potential political costs are probably of more immediate concern for the wife of former US president Bill Clinton.
She has struggled in surveys measuring her perceived trustworthiness and an active government investigation, especially one buoyed by evidence that top secret material coursed through her account, could negate one of her main selling points for becoming commander in chief – her national security record.
The Associated Press news agency learned ahead of the release that seven email chains would be withheld in full for containing “top secret” information.
The 37 pages include messages a key intelligence official recently said concerned “special access programmes” – highly restricted, classified material that could point to confidential sources or clandestine programmes like drone strikes.
“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 said, calling the withholding of documents in full “not unusual”.
That means they will not be published online with others being released, even with redactions.
Department officials would not describe the substance of the emails or say if Mrs Clinton sent any herself.
Mrs Clinton insists 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, but reviewers previously designated more than 1,000 messages at lower classification levels. The latest emails will be the first at top-secret level.
Even if Mrs Clinton did not write or forward the messages, she would still have been required to report any classification slippages she recognised in emails she received. But without classification markings, that may have been difficult, especially if the information was publicly available.
“We firmly oppose the complete blocking of the release of these emails,” Clinton campaign spokesman Brian Fallon said.
“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.”
Mr 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 before they ever reached Mrs Clinton and “in at least one case, the emails appear to involve information from a published news article”.
“This appears to be overclassification run amok,” Mr Fallon said.
Mr Kirby said the state department was focused, as part of a Freedom of Information Act review of Mrs Clinton’s emails, on “whether they need to be classified today”. Past classification questions, he said, “are being, and will be, handled separately by the state department”. It is the first indication of such a probe.
Department responses for classification infractions could include counselling, warnings or other action, officials said. They would not say if Mrs Clinton or senior aides who have since left government could face penalties.
Separately, Mr Kirby said the department withheld eight email chains, totalling 18 messages, between President Barack Obama and Mrs Clinton. These are remaining confidential “to protect the president’s ability to receive unvarnished advice and counsel” and will be released eventually like other presidential records.
The emails have been a Clinton campaign issue since 10 months ago, when the AP discovered her exclusive use while in office of a homebrew email server in the basement of her family’s New York home. Doing so wasn’t expressly forbidden. Clinton first called the decision a matter of convenience, then a mistake.
Last March Mrs Clinton and the state department said no business conducted in the emails included top-secret matters. Both said her account was never hacked or compromised, which security experts assess as unlikely.
Mrs Clinton and the state department also claimed the vast majority of her emails were preserved properly for archiving because she corresponded mainly with government accounts, but they have backtracked from that claim in recent months.
The special access programmes emails surfaced last week, when Charles McCullough, lead auditor for US intelligence agencies, told Congress he found some in Mrs Clinton’s account.
Mr Kirby confirmed the “denied-in-full emails” were among those Mr McCullough recently cited. He said one was among those Mr McCullough identified last summer as possibly containing top secret information.
AP reported last August that one focused on a forwarded news article about the CIA’s classified US drone programme. Such operations are widely discussed publicly, including by top US officials, and state department officials debated Mr McCullough’s claim. The other concerned North Korean nuclear weapons programmes, according to officials.
At the time, several officials from different agencies suggested the disagreement over the drone emails reflected a tendency to overclassify material and a lack of consistent classification policies across government.
The FBI also is looking into Mrs Clinton’s email set-up, but has said nothing about the nature of its probe. Independent experts say it is unlikely she will be charged with wrongdoing, based on details that have surfaced so far and the lack of indications that she intended to break laws.