The U.S. State Department said Friday that former Secretary of State Hillary Clinton's unsecured home server contained several email chains with top-secret information and that those emails would be withheld from public release.
Spokesman John Kirby said the material includes seven email chains totaling 37 pages. He said those and other emails from Clinton's archive would not be released.
Kirby said the documents were not marked classified at the time they were sent, but were being upgraded now at the request of the intelligence community.
The development came three days before the Iowa caucuses, the first event in which members of the public will cast ballots to determine their parties' next presidential nominees. Clinton has been the front-runner for the Democratic presidential nomination but is facing an increasingly close race in Iowa.
The Clinton campaign demanded Friday that the emails be released in full. A campaign statement said the development was an example of "bureaucratic infighting" and "overclassification run amok."
Clinton has been at the center of a controversy since it was discovered that she used her private email account for official business while serving as secretary of state.
Clinton has asked the State Department to release her emails in order to defuse the controversy, but it is taking officials months to sort through all the material to ensure there is no classified information in them.
State Department officials have said that using a private email account was not prohibited and that Clinton never shared classified information over the account. But critics have said it may have been a way for her to hide her communications.
Clinton initially said the private server was a matter of convenience, but later termed it a mistake.