The FBI has recommended no criminal charges against Hillary Clinton for her handling of classified information while she was secretary of state. Photo: The Political Insider

But 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. Comey’s 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.

As Mr. Comey strode to the lectern at the F.B.I. headquarters at 11 a.m., Mrs. Clinton was waiting backstage a few blocks away to address the National Education Association. Her aides said she did not know what Mr. Comey was going to say. Five minutes into Mr. Comey’s remarks, and before he announced that the F.B.I. would not seek charges, a smiling Mrs. Clinton began her speech.

By 2:45 p.m., she and Mr. Obama were descending the stairs of Air Force One for a campaign rally in Charlotte, N.C., while back in Washington the State Department spokesman, John Kirby, was fending off questions from reporters about what Mr. Comey described as the department’s lax security in handling classified information.

White House officials said Mr. Obama also did not know about Mr. Comey’s plans ahead of time. The F.B.I. director said he did not coordinate the statement with the Justice Department or any other agency. “They do not know what I am about to say,” he declared.

A Republican former federal prosecutor, Mr. Comey seemed at first to be laying the groundwork for some kind of legal charge. Speaking sternly, and in far more detail than he usually does, he listed several previously undisclosed findings from the F.B.I.’s investigation:

■ Of 30,000 emails Mrs. Clinton handed over to the State Department, 110 contained information that was classified at the time she sent or received them. Of those, Mr. Comey said, “a very small number” bore markings that identified them as classified. This finding is at odds with Mrs. Clinton’s repeated assertions that none of the emails were classified at the time she sent or received them. The F.B.I. did not disclose the topics of the classified emails, but a number of the 110 are believed to have involved drone strikes.

■ The F.B.I. discovered “several thousand” work-related emails that were not in the original trove of 30,000 turned over by Mrs. Clinton to the State Department. Three of those contained information that agencies have concluded was classified, though Mr. Comey said he did not believe Mrs. Clinton deliberately deleted or withheld them from investigators.

■ In saying that it was “possible” that hostile foreign governments had gained access to Mrs. Clinton’s personal account, Mr. Comey noted that she used her mobile device extensively while traveling outside the United States, including trips “in the territory of sophisticated adversaries.”

■ Mrs. Clinton used multiple private servers for her personal and government business, not just a single server at her home in New York that has been the focus of media reporting for more than a year. Her use of these servers — some of which were taken out of service and stored — made the F.B.I.’s job enormously complicated as it struggled to put together, in Mr. Comey’s words, a jigsaw puzzle with “millions of email fragments” in it.

Despite all that, Mr. Comey said the F.B.I. did not find that Mrs. Clinton’s conduct revealed “intentional misconduct or indications of disloyalty to the United States or efforts to obstruct justice.” But a person in her position, he said, “should have known that an unclassified system was no place” for the emails she was sending. And he said it raised troubling questions about how the State Department handled classified information.