The scandal over Hillary Clinton's use of a private email server while secretary of state returned on Wednesday to dog her presidential campaign.
A starkly critical report by the State Department's inspector general found she had not sought permission to conduct official business on her personal account.
Had she asked, she would not have been allowed to carry out government correspondence on her home-made set-up, for fear that state secrets could be hacked, the report said.
Clinton is still the frontrunner to secure the Democratic nomination in November's election to succeed Barack Obama in the White House.
But the email scandal continues to tarnish her main selling points as she prepares to take on Republican firebrand Donald Trump: experience and competence.
And there may yet be more revelations to come, as the FBI is conducting a separate investigation into whether state secrets were stolen or put at risk.
As the report was made public, Clinton's camp pushed back hard, insisting it had found she had acted as previous secretaries of state had done before her.
"Hillary Clinton's use of personal email was not unique," campaign spokesman Brian Fallon said.
"And she took steps that went much further than others to appropriately preserve and release her records."
The inspector general report does indeed note that Clinton's Republican predecessor Colin Powell also used a private email account.
But it makes it clear that when Clinton joined the State Department in 2009, official guidelines had been updated and that she should have known about them.
"Secretary Clinton's cybersecurity practices accordingly must be evaluated in light of these more comprehensive directives," the report said.
All in all, under Clinton and three of her predecessors, the audit found "systemic weaknesses related to electronic records and communications."
When Clinton took office, the department's Foreign Affairs Manual said normal day-to-day operations should be conducted on an authorized system.
Yet the report "found no evidence that the secretary requested or obtained guidance or approval to conduct official business via a personal email account."
The State Department's current heads of information security told the inspector general that Clinton had a duty to ask permission to use her personal email.
And they said that, had they been asked they would not have granted Clinton permission to do so, because of the rules in place and the "security risks."
State staff must seek guidance from the Bureau of Information Resource Management before sending sensitive information outside the department.
But the investigation found no evidence Clinton "ever contacted IRM to request such a solution."
And this happened "despite the fact that emails exchanged on her personal account regularly contained information marked as SBU" -- or sensitive but unclassified.
Some senior State Department employees interviewed for the report admitted there was "some awareness" that Clinton was using a private server.
Because Clinton wanted to use her BlackBerry portable phone in secure areas -- a cybersecurity no-no -- there was talk of getting her a networked terminal.
In November 2010, officials discussed setting up an official State Department account for Clinton to use in parallel to her private mailing address.
In response, Clinton wrote: "Let's get a separate address or device but I don't want any risk of the personal being accessible."
She never obtained a State Department email account and, later in 2011, discussions about getting her a secure official BlackBerry came to nothing.
The communications of senior US officials are matters of public record and can be requested through freedom of information legislation.
When Clinton submitted to this after leaving her post her family lawyers first sifted through the server and removed thousands of mails deemed personal.
The remaining 30,000 mails were examined by the State Department, and hundreds were found to contain information that ought to have been classified.
The rest were released, but some Clinton critics continue to allege that she used the private server to conceal embarrassing facts from voters.
Trump has taken to calling Clinton "crooked" in tweets to his supporters, and Republican chairman Reince Priebus issued a strongly worded statement.
"The stakes are too high in this election to entrust the White House to someone with as much poor judgment and reckless disregard for the law," he said.