An internal audit found Democratic presidential front-runner Hillary Clinton responsible for violating secrecy laws by accessing an official server from her house.
Clinton broke government rules by using a private email server without approval during her time as US Secretary of State, the report suggests.
The long-awaited report by the US State Department was made public on Wednesday and mentions ‘longstanding, systemic weaknesses related to electronic records and communications within the office of the secretary.'
The report contradicted Clinton's repeated assertion that she was allowed to use the server and that no permission was needed.
The report found problems with the email record-keeping and it cited that Clinton’s predecessors, particularly Colin Powell had also failed to comply with the Federal Records Act.
The report concluded that Clinton would not have been allowed to use the server in her home had she asked the department officials in charge of information security.
The report said that staff who later raised concerns about the incident were told to keep quiet.
Several suspected hacking attempts in 2011 were never reported to department information security officials, in breach of department rules, reads the report.
The inspector general's office examined email record-keeping under five US Secretaries of State, both Democratic and Republican.
John Kerry, the current officeholder and predecessors Madeline Albright, Colin Powell and Condoleezza Rice all agreed to speak to the inspector general's investigators. Clinton was the only one who declined to be interviewed.
Trump criticises Clinton
"She's as crooked as they come," Donald Trump, the Republican presidential candidate, said of Clinton at a campaign rally in California. Making a point against his political rival, Trump said that the report's findings were "not good" for her.