The department's investigative office made the conclusion in a report released only to members of Congress and obtained by ABC News. The report examined the email practices of five U.S. secretaries of state and found that there was "a limited ability to retrieve email records, inaccessibility of electronic files, failure to comply with requirements for departing employees and a general lack of oversight."
Clinton's campaign for president has been dogged by questions surrounding last year's revelation that she used a private email server to send official correspondence during her entire tenure as secretary of state. She has since turned over many of the messages from her private account but deleted others she deemed irrelevant to her professional work.
The FBI is investigating the handling of sensitive information on that server to determine whether there was any criminal wrongdoing.
The report comes in response to questions from Capitol Hill about the email practices of the current and past four secretaries of state and their immediate staffs.
About Clinton specifically, the report says that she should have preserved federal records she created and received on her personal account and that sending emails from the personal account to other employees at the department was "not an appropriate method of preserving" federal records.
The report also said the inspector general "found no evidence that the Secretary requested or obtained guidance or approval to conduct official business via a personal email account on her private server," and that she did not meet her "obligation to discuss using her personal email account to conduct official business." Had she done so, the report says, the relevant management offices "would not approve her exclusive reliance on a personal email account to conduct Department business, because of the restrictions in the FAM [Foreign Affairs Manual] and the security risks in doing so."
Secretary of State John Kerry and former Secretaries Madeline Albright, Colin Powell and Condoleezza Rice participated in interviews with the inspector general’s office, but Clinton and her aides denied requests to be interviewed.
Clinton's campaign spokesman, Brian Fallon, released a statement this afternoon accusing Clinton's political opponents of misrepresenting the report for "partisan purposes." In reality, he said, "the inspector general documents just how consistent her email practices were with those of other secretaries and senior officials at the State Department who also used personal email."
Rep. Elijah Cummings of Maryland, the ranking Democrat on the House Oversight and Government Reform Committee, said the report shows that Clinton was not the only secretary to use private email and that she is the only one who has turned them over.
"While Secretary Clinton preserved and returned tens of thousands of pages of her emails to the department for public release, Secretary Powell returned none," Cummings said in an emailed statement. "Republicans need to stop wasting taxpayer dollars singling out Secretary Clinton just because she is running for president."
Cummings also pointed out that today's report does not accuse Clinton of any crime.
Rep. Jason Chaffetz of Utah, the Republican chairman of that committee, suggested that Clinton took advantage of the State Department's lax enforcement of email policy. "Those weaknesses may have been exploited by department officials for self-serving purposes,” he said in a statement to the press.