The FBI will not recommend criminal charges in its investigation into Hillary Clinton's use of a private email server while US secretary of state, the agency's director has said.
James Comey made the announcement three days after FBI agents interviewed Mrs Clinton in a final step of its investigation.
The US Justice Department has been looking into whether anyone mishandled classified information that flowed through Mrs Clinton's email server.
Attorney General Loretta Lynch said last week that she would accept the recommendations of Mr Comey and of career prosecutors.
Although Mr Comey's announcement removes the threat of criminal charges, it is unlikely to eliminate concerns about Mrs Clinton's trustworthiness.
And it almost certainly will not stop Republican presidential candidate Donald Trump from continuing to make the server a campaign issue.
Mr Comey said although the investigation found "extremely careless" behaviour by Mrs Clinton and her staff in their handling of sensitive information, the FBI had concluded that "no charges are appropriate".
He said the agency believed that "no reasonable prosecutor would bring such a case".
Mr Comey's decision almost certainly brings the legal part of the issue to a close and removes the threat of criminal charges.
Mrs Clinton's personal email server, which she relied on exclusively for government and personal business, has dogged her campaign since its existence came to light in March 2015.
She has repeatedly said that no email she sent or received was marked classified, but the Justice Department began investigating last summer following a referral from the inspector general for the State Department and the intelligence community.
The scrutiny was compounded by a blistering audit in May from the State Department's inspector general, the agency's internal watchdog, which said that Mrs Clinton and her team ignored clear warnings from department officials that her email set-up violated federal standards and could leave sensitive material vulnerable to hackers.
Mrs Clinton declined to talk to the inspector general, but the audit said that she had feared "the personal being accessible" if she used a government email account.
The Clinton campaign said agents interviewed her on Saturday at FBI headquarters. Agents had earlier interviewed top Clinton aides including her former State Department chief of staff, Cheryl Mills, and Huma Abedin, a long-time aide who is now the vice chairwoman of Mrs Clinton's campaign.
Mrs Lynch said she would accept whatever findings and recommendations were presented to her. Though she said she had already settled on that process, her statement came days after an impromptu meeting with Bill Clinton on her plane in Phoenix that she acknowledged had led to questions about the neutrality of the investigation.
Mr Comey said the FBI found that 110 emails were sent or received on Mrs Clinton's server containing classified information.