WASHINGTON — A yearslong State Department investigation into former Secretary of State Hillary Clinton’s private email server found that while the use of the system for official business increased the risk of compromising classified information, there was no systemic or deliberate mishandling of classified information.
The inquiry, started more than three years ago, found that 38 current or former State Department officials were “culpable” of violating security procedures in a review of about 33,000 individual emails sent to or from the server that Ms. Clinton turned over to investigators.
The nine-page unclassified report, completed last month and shared with Congress this week, appears to bookend a controversy that dogged Ms. Clinton’s 2016 presidential campaign against Donald J. Trump. Ms. Clinton blamed the F.B.I.’s handling of the inquiry for crippling her campaign after James B. Comey, then the bureau’s director, reopened his investigation into the server days before the general election after initially declining to bring charges.
“While there were some instances of classified information being inappropriately introduced into an unclassified system in furtherance of expedience,” the report said, “by and large, the individuals interviewed were aware of security policies and did their best to implement them in their operations.”
The report concluded, “There was no persuasive evidence of systemic, deliberate mishandling of classified information.”
A representative for Ms. Clinton declined on Friday to comment on the report.
State Department investigators reviewed thousands of pages of documents, sent from 2009 to 2013, when Ms. Clinton served in the Obama administration. The emails were on subjects that were not considered classified at the time, but that have been or were retroactively marked as classified. Investigators also took statements from hundreds of past and present department officials, and interviewed dozens more, according to the report.
The focus of the investigation, according to the report, was to determine if any of the emails “represented a failure to properly safeguard classified information,” and if so, if any specific individuals could be determined to bear responsibility.
It is unclear whether any of the 38 officials the report found culpable of violating security procedures still work at the State Department. The office overseeing diplomatic security would make a note in the file of employees or former employees found to have violated those procedures, and those who have left government could have a harder time receiving security clearances if they apply for a government job again.
The use of a private email system “did not necessarily” increase the likelihood of classified information being transmitted on unclassified systems, investigators concluded, but it did add “an increased degree of risk of compromise.”
The report was released this week by Senator Charles E. Grassley, Republican of Iowa, who began investigating Ms. Clinton’s use of a private email server in 2017, when he served as the chairman of the Senate Judiciary Committee.
Not included in the review were emails deleted by Ms. Clinton after her lawyers advised that they were private and not related to official business.
House Republicans discovered the use of the private server, which Ms. Clinton said she used out of convenience, as part of an investigation into the deaths of Americans in Benghazi, Libya. It prompted Congress, the F.B.I. and the State Department to initiate separate investigations.
Some Republicans, led by Mr. Trump, have continued to use the issue to attack Ms. Clinton and Democrats, even though the F.B.I. closed a previous investigation without bringing charges against Ms. Clinton, Critics have accused State Department officials who took up the investigation of political bias.
Mr. Trump’s own administration officials — including his daughter Ivanka Trump and son-in-law Jared Kushner — have admitted to using private messaging services to conduct official work. House Democrats’ impeachment inquiry has revealed that Trump administration diplomats used private phones to message each other about their efforts to pressure Ukrainian officials to investigate the president’s American political rivals, including the Bidens.
Maggie Haberman and Edward Wong contributed reporting.