Democratic presidential hopeful Hillary Clinton did not comply with the state department's policies in her use of private email for work while secretary of state, an official audit has found.
A report made public on Wednesday by the state department's independent inspector said Clinton and previous top US diplomats were poorly managing information and slowly responding to new cyber security risks.
"At a minimum, Secretary Clinton should have surrendered all emails dealing with department business before leaving government service and, because she did not do so, she did not comply with the Department's policies that were implemented in accordance with the Federal Records Act," the report read.
Clinton, who was secretary of state from 2009 to 2013, declined to be interviewed for the inspector general's investigation, US media outlets reported on Wednesday.
Brian Fallon, Clinton's spokesman, said in a statement that the report shows her electronic record-keeping systems "were longstanding" and emphasises that her use of a private email server "was known to officials within the department during her tenure".
Fallon acknowledged that "steps ought to have taken" to better maintain official records.
Al Jazeera's Shihab Rattansi, in Washington DC, said the report was likely to make voters trust her even less than they already do.
"This is going to add to that sense that she says one thing and does another," he said. "For those who already don't trust Hillary Clinton it is just going to add much more of a suggestion that she shouldn't be trusted."
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She used a private server set up for her home and used it for work and personal emails, including some that have since been classified.
Mark Toner, the state department spokesman, said on Wednesday that the agency was "already working" to improve its email and records management system.
"It is clear that the department could have done a better job preserving emails and records of secretaries of state and their senior staff going back several administrations," he said.
The Federal Bureau of Investigation is investigating whether any laws were broken as a result of the server kept in Clinton's home.