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 records, the department’s inspector general found, and used private email for official business without approval.
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.
Security flaws have long plagued the department, the report finds.
There are “longstanding, systemic weaknesses” in electronic record-keeping, it said, “that go well beyond the tenure of any one secretary of state”.
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.