The FBI has recommended that no criminal charges be filed over Hillary Clinton's use of private email servers while she was secretary of state.
The Bureau however sharply rebuked the presumptive Democratic US presidential candidate for "extremely careless" handling of classified information.
FBI director James Comey said the year-long investigation into the server found that at least 110 emails contained classified information at the time they were sent.
But he said the FBI's judgement was that "no reasonable prosecutor" would bring charges against Mrs Clinton, who had a voluntary three-and-a-half hour interview with the FBI in Washington last Saturday.
"Although there is evidence of potential violations of the statutes regarding the handling of classified information, our judgement is that no reasonable prosecutor would bring such a case," Mr Comey told a news conference in Washington.
"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," he said.
However, Mr Comey has said there was "evidence of potential violations of the statuses regarding the handling classified information."
The FBI had been investigating whether Mrs Clinton broke the law as a result of a personal email server kept in her New York home while she was secretary of state from 2009 to 2013. One of the questions is whether she mishandled classified information on the server.
The investigation dogged her campaign for the past year, as she and her staff struggled to respond to accusations that her use of the server in violation of State Department protocol indicated she was untrustworthy and considered herself above the law.
Republican presidential candidate Donald Trump has said the investigation should disqualify Mrs Clinton from being president, and Mr Comey's strong criticism of her judgement is likely to fuel a continued onslaught on the issue by Mr Trump.
The Clinton presidential campaign said it was "pleased" over the FBI's decision.
"We are pleased that the career officials handling this case have determined that no further action by the (Justice) Department is appropriate," her campaign spokesman Brian Fallon said in a statement.
"As the secretary has long said, it was a mistake to use her personal email and she would not do it again. We are glad that that this matter is now resolved".
'I believe in Hillary Clinton,' Obama tells voters
President Barack Obama has meanwhile threw his full weight behind Mrs Clinton's bid to succeed him, extolling the experience and fighting spirit of his former secretary of state at their first joint campaign appearance.
"I'm here today because I believe in Hillary Clinton," Mr Obama told the rally in North Carolina taking place just hours after the FBI's announcement.
Mrs Clinton travelled together with Mr Obama on Air Force One for the first in a series of high-profile events intended to give the presumptive Democratic nominee a boost in her November showdown with Donald Trump.
Obama takes ‘everybody can tweet’ swipe at Trump as he backs Hillary Clinton to 'sit behind the desk'https://t.co/6VQTf6A5FH
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"There has never been any man or woman more qualified for this office,” Mr Obama told the crowd.
"She is and will be a stateswoman who makes us proud around the world," Mr Obama said. "I am ready to pass the baton. And I know that she is going to take it."
Neither Mrs Clinton nor Mr Obama made any mention of the FBI's recommendation to the Justice Department.
