Likely 2016 U.S. presidential candidate Hillary Clinton used a personal email account to conduct official business as secretary of state, according to the State Department.
The New York Times reported the revelation, saying Clinton may have violated federal records laws that require archiving official government documents.
"Her aides took no actions to have her personal emails preserved on department servers at the time, as required by the Federal Records Act," the newspaper reported.
The Times said Clinton "exclusively" used a private account during her four years at the State Department and did not have a government email address.
The report is drawing even more scrutiny because of Clinton's expected place in the 2016 presidential election. She is widely seen as the frontrunner for the Democratic Party nomination.
Clinton is not the only secretary of state to conduct official business via private email, but political analysts say doing so for all correspondence is unusual. In a statement issued in response to the article, State Department spokeswoman Marie Harf said current Secretary of State John Kerry is the first "to rely primarily on a state.gov email account."
A spokesman for Clinton, Nick Merrill, told the Times she has complied with the "letter and spirit of the rules," and that she had "every expectation" her emails with other State Department officials "would be retained."
Harf, meanwhile, said the State Department "has long had access to a wide array of Secretary Clinton's records, including emails between her and Department officials with state.gov accounts."
"Last year, the Department sent a letter to representatives of former secretaries of state requesting they submit any records in their possession for proper preservation as part of our effort to continually improve our records preservation and management," said Harf. "In response to our request, Secretary Clinton provided the Department with emails spanning her time at the Department."
The Times said Clinton's personal email account was discovered by a House committee investigating the 2012 terrorist attack on the U.S. consulate in Benghazi, Libya, in which four Americans, including U.S. Ambassador to Libya Chris Stevens, died. Clinton was secretary of state at the time and faced scrutiny over her handling of the incident.
After reviewing the emails Clinton turned over, the State Department said it provided about 300 emails to the committee.
The news of Clinton's private emails quickly generated responses on Twitter. "Transparency matters," tweeted former Florida Governor Jeb Bush, potential Republican presidential candidate and brother of former President George W. Bush. "Unclassified @HillaryClinton emails should be released," he added. Bush released tens of thousands of his own emails from his time as governor in December and included a link to them in his post about Clinton's emails.
The vulnerability of personal email to hackers is another concern. The Times said "it is not clear whether Mrs. Clinton's private email account included encryption or other security measures, given the sensitivity of her diplomatic activity."