WASHINGTON — The F.B.I. director, James B. Comey, said on Tuesday that the F.B.I. was recommending no charges against Hillary Clinton for her use of a personal email server while secretary of state.
Mr. Comey said that “no reasonable prosecutor” would bring a case against Mrs. Clinton for her handling of classified information on her private server. But he said Mrs. Clinton and her staff were “extremely careless” in their use of email.
The F.B.I.’s recommendation to the Justice Department will have an enormous impact on the presidential election.
Attorney General Loretta Lynch said on Friday that she would accept whatever recommendation she received from the F.B.I.
Mrs. Clinton is scheduled to campaign with President Obama in North Carolina on Tuesday afternoon, their first joint appearance of her presidential bid.
The statement by Mr. Comey concluded an investigation that began a year ago when the inspector general for the intelligence agencies told the Justice Department that he had found classified information among a small sampling of emails Mrs. Clinton had sent and received.
The inspector general, I. Charles McCullough III, said that the emails contained information that was classified at the time they were sent but were not marked classified, and that the information should never have been sent on an unclassified system.
The discovery of Mrs. Clinton’s email practices grew out of a request by the House Select Committee on Benghazi for communications between Mrs. Clinton and other officials surrounding the September 2012 attack on the diplomatic outpost in Benghazi, Libya, that killed four Americans, including Ambassador J. Christopher Stevens.
As lawyers for the State Department gathered materials, they discovered that Mrs. Clinton had used a personal, nongovernment address for her email and routed the messages through a server, kept in her home in Chappaqua, N.Y.
After a negotiation between the State Department and Mrs. Clinton’s lawyers, she agreed to turn over 55,000 pages of email from her time as secretary of state. She withheld email — roughly half the total number of messages — that she said touched on personal issues, from yoga classes to the flower arrangements for her daughter’s wedding.
The State Department turned over to the House committee roughly 800 emails pertaining to Benghazi. Mrs. Clinton asked the department to release the remaining trove of emails, which set off a complicated, politically charged process of vetting each one to determine whether it contained classified information.
The C.I.A., the State Department and other agencies reviewed the emails, designating hundreds of them with varying levels of classification.
Mrs. Clinton has asserted that she did not send or receive any information marked classified at the time it was sent. But about two dozen emails were designated “top secret,” the highest level of classification, and Mrs. Clinton’s critics say she jeopardized national security.
Several of those pertained to the C.I.A.’s drone program in Pakistan, which is a covert program, though it is widely reported in the Pakistani and American news media.