A State Department review has faulted Hillary Clinton for her exclusive use of a private email server during her tenure as Secretary of State, the report said on Wednesday.
It said that it posed a security risk and did not comply with government law record.
Revelations that Clinton used a personal email address and a server located in her home had raised questions about her tenure at the State Department and dogged her presidential campaign for months.
The report by the State Department Office of Inspector-General found Clinton had not sought permission for her unusual email arrangement and would likely not have been granted permission had she done so.
It also detailed warnings to Clinton’s aides about problems with the email arrangement.
The report was due to be publicly released on Thursday, but the Washington Post published a copy of it on its website Wednesday.
The U.S. State Department had released thousands of Clinton’s emails in 2015, and earlier this year in order to comply with a judge’s order in the face of lawsuits seeking their release under open records laws.
The State Department began making the emails public in May, after revelations that Clinton used a private server rather than a government account to send emails while serving as Secretary of State.
Clinton had said using the private server was a mistake, as the email issue had become a liability in her presidential campaign.
It had raised questions about transparency, technical security and about her handling of sensitive email relating to the deadly 2012 attack on the US diplomatic compound in Benghazi, Libya.
The FBI is also investigating Clinton’s server to see if classified information was mishandled.
In addition, several newspapers and other media outlets as well as Republicans in Congress want to examine the emails, as her years of service come under the microscope of a presidential race.
Clinton is the likely Democratic nominee for president, but still faces a primary challenge from Vermont Senator Bernie Sanders.
Sanders had vowed to remain in the race in spite of trailing Clinton in the number of delegates needed to secure the nomination.