A state department inquiry has accused Hillary Clinton and other former US secretaries of state of poorly managing email security.
Mrs Clinton failed to comply with rules on record-keeping, the inspector general found, and used private email for official business without approval.
But there are “longstanding, systemic weaknesses” in record-keeping that pre-dated her tenure, it said.
Mrs Clinton said the report showed she was no different from her predecessors.
“Contrary to the false theories advanced for some time now, the report notes that her use of personal email was known to officials within the department during her tenure,” said a statement from the campaign.
It added that there was no evidence of any successful breach of her server.
The FBI is also investigating whether there was a security breach and but has not yet interviewed her.
She has denied handling any classified information in her private emails.
Analysis – Anthony Zurcher, BBC News, Washington
The Clinton email story has been at a steady drip for over a year now, as new revelations are unearthed and her electronic correspondence is released. This critical State Department report is bigger than that – more akin to a deluge of bad news for the candidate.
It will have a political toll, but it may be the first step in her putting the issue behind her. It comes when she is least politically vulnerable, near the end of a primary campaign she comfortably leads and before July’s political conventions.
Mrs Clinton’s critics will point to the findings that she did not get permission from the State Department to set up her email server, her recordkeeping was inadequate, the information she disclosed was incomplete, and she and her aides were not always co-operative in the investigation.
The Clinton campaign, on the other hand, has been quick to point out that the actions of previous secretaries of state were also highlighted in the report – although the inspector general noted that the guidelines were more detailed by the time Mrs Clinton took office. She was was also not directly told by the State Department to stop using her system.
With this report now public, the last remaining key piece of the story is also the potentially most damaging – the FBI’s investigation into whether her handling of classified information constituted criminal conduct.
Although early indications are that it did not, an adverse ruling there would amount to a flood that could sweep away her presidential hopes.
The likely Democratic presidential nominee’s use of private email has been a talking point in the election, with critics saying Mrs Clinton believes she is above the law.
Mrs Clinton’s campaign says it showed that her practices were consistent with those of other secretaries of state who “also used personal email” and she was “not unique” in doing it.
State Department spokesman Mark Toner said that the report makes clear that the agency could have done a better job preserving emails and records, going back several administrations.