The US state department on Friday released 296 emails -- out of a total of 30,000 -- that then secretary of state Hillary Clinton sent and received about a 2012 attack on the US consulate in Benghazi, Libya.
US ambassador to Libya J Christopher Stevens and three other Americans were killed in the attack that took place on the anniversary of the 9/11 attacks.
The emails, exchanged by Clinton with her senior aides at the state department, were with a congressional committee, investigating the Benghazi attack, since February.
The mails do not deal directly with the events leading to the attack and contain mostly press reports, drafts of speeches, comments and some administrative details.
Clinton's emails have come under scrutiny because she was using a account hosted on a private server, and not a government account, as is mandatory.
"These documents span a two-year period from January 1, 2011 through December 31, 2012 and relate to the security of, and attacks on, the state department facility in Benghazi and to the United States' diplomatic presence in Libya, including in Benghazi," state department spokesperson Marie Harf said in a statement.
She added: "The emails we release today do not change the essential facts or our understanding of the events before, during, or after the attacks, which have been known since the independent Accountability Review Board report on the Benghazi attacks was released almost two and a half years ago."
Republicans have tried to portray the Benghazi attack as a failure of Barack Obama's administration and first tried to blame the President for it during his re-election campaign.
The emails indicate Clinton received information on the deadly attack that has now been classified.
The email in question, forwarded to Clinton, the presumed Democratic presidential candidate in 2016, by her deputy chief of staff, Jake Sullivan, relates to reports of arrests in Libya of possible suspects in the attack.
Because the information was not classified at the time the email was sent, no laws were violated. But Friday's redaction shows that Clinton received information considered sensitive on her unsecured personal server, which came to light just as she was beginning her presidential campaign.
Clinton, campaigning in the early primary voting state of New Hampshire, said on Friday she was aware that the FBI wanted some of the email to be classified, "but that doesn't change the fact all of the information in the emails was handled appropriately."
Asked if she was concerned it was on a private server, she replied, "No."
The information was not classified at the time the email was sent but was upgraded from "unclassified" to "secret" on Friday at the request of the FBI, according to State Department officials, speaking on condition of anonymity because they weren't authorised to discuss the decision.
They said 23 words of the November 18, 2012, message were redacted from Friday's release of 296 emails totaling 896 pages to protect information that could harm national security and damage foreign relations.
Officials who received the email have been informed that the 23 words are now classified as "secret" and that they should take appropriate measures to protect it in any files they may have, the officials said.
No other redactions were made to the collection of Benghazi-related emails for classification reasons, the officials said.