Comey's comments are likely to reinforce what polls show are widespread public concerns about Clinton's honesty and trustworthiness.
While FBI Director James Comey's announcement lifted a cloud of uncertainty that had loomed over Clinton's White House campaign, his strong criticism of her judgment ignited a new attack on her over the email issue by Donald Trump, her likely Republican opponent in the November 8 election.
Comey's comments are likely to reinforce what polls show are widespread public concerns about Clinton's honesty and trustworthiness. Republicans have pointed to the controversy as evidence that she considered herself above the law.
In a lengthy statement on the Federal Bureau of Investigation's conclusions from its yearlong investigation, Comey directly contradicted statements Clinton has made while defending her use of the private email setup.
He said, for example, that the FBI found at least 110 emails that contained classified information when they were sent, although Clinton has repeatedly said she never sent or received classified information on her private servers.
"Although we did not find clear evidence that Secretary Clinton or her colleagues intended to violate laws governing the handling of the classified information, there is evidence that they were extremely careless in their handling of very sensitive, highly classified information," Comey said.
But he said the FBI concluded "no reasonable prosecutor" would bring charges. "Although the Department of Justice makes final decisions on matters like this, we are expressing to Justice our view that no charges are appropriate in this case," Comey told reporters in Washington.
His recommendation will likely stand. The country's top prosecutor, U.S. Attorney General Loretta Lynch, said on Friday she would accept the recommendations of career prosecutors and the FBI director on whether to charge Clinton for mishandling emails.