Hillary Clinton was "careless" when handling her emails but the FBI would not recommend that charges should be brought against her, FBI Director James Comey said.
The Democratic presidential contender has been the subject of a long-running controversy over her use of private email while she was Secretary of State.
WikiLeaks yesterday released more than 1,000 emails from Hillary Clinton’s private email server pertaining to the Iraq War.
Mr Comey said that the investigation showed that 110 emails in 52 email chains were determined to include classified information at the time they were received.
Mr Comey said there was evidence of "extremely careless handling" of Clinton's emails.
He said "any reasonable person" should "have known an unclassified system was no place for that conversation."
It is possible that hostile actors gained access to Hillary Clinton's personal email account, he added.
However, he added the FBI did not find clear evidence that she or her colleagues intended to violate laws governing classified information.
The agency director said: "It is our judgement that no reasonable prosecutor would bring a case against Clinton for the emails".
He said: "We are expressing to the Justice Department that no charges should be brought."
ITV News Washington Correspondent Robert Moore reports
Clinton's spokesman said the campaign is pleased with the FBI decision not to recommend charges after the email probe.
The spokesman said the use of personal email while at the State Department was a "mistake" but the matter is now considered to be resolved.
Speaker Paul Ryan said the decision not to prosecute Clinton defies explanation and "no one should be above the law".