Ms. Clinton has said she didn’t send or receive information that was classified at the time via her personal email account, which was run on a private server at her New York home. Republicans have repeatedly questioned whether her use of a private email system put sensitive information at risk.

In all, the State Department on Thursday said 1,274 of Ms. Clinton’s emails have been retroactively classified since the department started reviewing them for public release.

Two emails released on Thursday were designated “secret,” the second-highest level of classification, which applies to information that could cause serious damage to national security if released. Most of the emails were classified “confidential,” which is the lowest level of classification.