The US State Department says its former chief Hillary Clinton broke federal rules by using her own personal email for communicating sensitive materials.
The department said Clinton ignored clear directives as she never sought approval to conduct government business over private email.
The findings of an inquiry by the US inspector general on Wednesday also revealed that hacking attempts forced Clinton off email at one point in 2011, though she insists the personal server she used was never breached.
Clinton and several of her senior staff declined to be interviewed for the investigation.
Critics have questioned whether her email server might have made a tempting target for hackers, especially those working with or for foreign intelligence services.
Earlier this month, a US federal judge in Washington said Clinton might be ordered to testify under oath about her private email server.
The State Department has released more than 52,000 pages of Clinton’s work-related emails, including some that have since been classified. Clinton has withheld thousands of additional emails, saying they were personal.
The FBI has already interviewed some of Clinton’s aides, including Huma Abedin, the vice chairwoman of Clinton’s presidential campaign, over the email practice.
A nine-month FBI investigation into whether laws were broken as a result of the server kept in her New York State home has overshadowed Clinton’s campaign.
More than 2,000 emails sent and received by Clinton while working as the top US diplomat between 2009 and 2013 include classified information, which the government bans from being handled outside secure, government-controlled channels.
Critics, including likely Republican presidential election rival Donald Trump, say she endangered government secrets and evaded transparency laws.