On a day of political high drama in Washington, Mr. Comey rebuked Mrs. Clinton as being extremely careless in using a private email address and server. He raised questions about her judgment, contradicted statements she has made about her email practices, said it was possible that hostile foreign governments had gained access to her account, and declared that a person still employed by the government Mrs. Clinton left the State Department in 2013 could have faced disciplinary action for doing what she did.

To warrant a criminal charge, Mr. Comey said, there had to be evidence that Mrs. Clinton intentionally transmitted or willfully mishandled classified information. The F.B.I. found neither, and as a result, he said, our judgment is that no reasonable prosecutor would bring such a case.

The Justice Department is highly likely to accept the F.B.I.s guidance, which a law enforcement official said also cleared three top aides of Mrs. Clinton who were implicated in the case: Jake Sullivan, Huma Abedin and Cheryl D. Mills. Attorney General Loretta Lynch said last week that she would accept the recommendation of the F.B.I. and career prosecutors in the case after a storm of criticism about an impromptu meeting between her and former President Bill Clinton on an airport tarmac in Phoenix.

Mr. Comeys 15-minute announcement, delivered with no advance warning only three days after his investigators interviewed Mrs. Clinton in the case, riveted official Washington and is likely to reverberate for the rest of the campaign. In offices across the capital, all eyes turned to television screens to hear the outcome of a yearlong investigation that could have thrown the 2016 presidential election into disarray and changed history.