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 Mrs. 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 Mrs. Clinton’s 2016 presidential campaign against Donald J. Trump. Mrs. 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.”