Hillary Clinton forwarded a chain of emails in 2009 to her daughter, Chelsea, that included information, most likely about climate talks, that the State Department later declared to be classified, according to a new batch of emails released on Friday.
The email chain was among thousands of messages that the F.B.I. uncovered as part of its yearlong investigation into Mrs. Clinton’s handling of classified information on the private email server she used while secretary of state.
The State Department had previously released parts of the chain — which involved senior members of the White House, Treasury Department and State Department — with the information entirely redacted. The disclosure on Friday appeared to be the first to show that Mrs. Clinton had forwarded information of a delicate nature to her daughter, who had an email address on her server under a pseudonym.
The chain, among 285 pages of emails released by the State Department on Friday, renewed attention on Mrs. Clinton’s handling of classified material, an issue that has dogged her campaign from its inception and has given fodder to her Republican rival, Donald J. Trump. The F.B.I. reignited the issue last Friday when its director, James B. Comey, informed Congress that investigators had uncovered additional emails to and from a close Clinton aide, Huma Abedin, on the computer of her estranged husband, Anthony D. Weiner.
Investigators are scouring those emails and have not yet said whether they are pertinent to an inquiry that seemingly ended in July when Mr. Comey announced that he would not recommend criminal charges against Mrs. Clinton or others over their handling of classified information.
The chain released on Friday began with an email, dated Dec. 19, 2009, with the subject line “Update,” from Michael B. Froman, then a deputy for economic affairs on the National Security Council. It was not marked classified at the time and was sent on the government’s unclassified networks.
Like information in more than 2,000 other emails that have been made public, two of the notes in the chain were “upgraded” by the State Department — on the grounds that they contained information classified at the lowest level, “confidential” — when they were released last October. The newest link in the chain was Mrs. Clinton’s forwarding of the note to her daughter, who used an email address under the name Diane Reynolds. “See below,” Mrs. Clinton wrote.
Even if the information was not classified at the time, it would be unusual for an exchange among senior officials to be forwarded to someone outside the government, even a close relative. Steven Aftergood, an expert on secrecy for the Federation of American Scientists, said it was difficult to say whether it was inappropriate without knowing the information contained in the chain.
A spokesman for Mrs. Clinton’s campaign, Brian Fallon, responded to questions about the email chain with a statement noting only that the State Department had changed the classification of the information when it was prepared for release last year.
The timing of the chain and the people copied on the emails almost certainly meant that it was related to the Copenhagen climate change talks, which culminated that day in December 2009. During the conference, Mrs. Clinton and President Obama crashed a meeting of officials from major emerging economies and extracted an agreement to monitor global carbon emissions, which rescued the negotiations.
The forwarded chain was among 431 emails sent either to or from Mrs. Clinton, totaling more than 1,500 pages, that the State Department released over two days this week. A federal court ordered the State Department to produce the new emails — many of which are duplicates of previously released emails — on a rolling basis both before and after the election.
A majority of the new emails reflected routine scheduling matters, but others offered granular and occasionally interesting details of Mrs. Clinton’s tenure as secretary of state. Ms. Abedin’s husband factored into an exchange in 2010, in which she discussed sending a “secure” phone via FedEx to Mrs. Clinton or finding another solution. “Maybe one of Anthony’s trusted staff could deliver secure phone,” Mrs. Clinton suggested.
After that email was released on Thursday, the State Department issued a statement saying that a phone configured to handle secure communications could in some circumstances be sent by courier or through commercial delivery companies.
Michael Short of the Republican National Committee said in a statement that the hundreds of emails now deemed to have classified information were “a stark reminder” of Mrs. Clinton’s “reckless conduct that jeopardized our national security and sensitive diplomatic efforts.”
