Hillary Clinton used a personal email account during the four years she served as U.S. Secretary of State, which possibly violates rules requiring federal officials preserve their emails for government records, The New York Times reported.

Not only did Clinton send emails using a private account instead of a government issued one, but she conducted all State Department business using solely that account, which record-keeping officials said was odd and considered a possible breach of the Federal Records Act.

"It is very difficult to conceive of a scenario - short of nuclear winter - where an agency would be justified in allowing its cabinet-level head officer to solely use a private email communications channel for the conduct of government business," Jason R. Baron, a lawyer and former director of litigation at the National Archives and Records Administration, told The NY Times.

All officials are required to provide email records to the National Archives and Records Administration, according to agency rules at the time, so they can be later accessed by other agencies, historians or journalists. There are few exceptions for using a private email account, such as when dealing with classified information or if they agency's computers are down.

Neither Clinton nor her staff provided email records during her time in office from 2009 to early 2013, the newspaper reported.

Clinton's usage of a private account was discovered during a House committee investigation into communications between Clinton and her aids about the 2012 attack on the American Consulate in Benghazi, Libya.

The State Department later requested Clinton's aides turn over her email records, comprised of tens of thousands of pages, as part of a larger effort to comply with federal laws. A total of 55,000 pages of the former secretary's emails were turned over, The NY Times reported. It's not clear exactly how many emails were in her account.

It's also unclear why she used a private email account while conducting State Department business. She is not the first federal official or secretary of state to do so, but experts noted her case is unusual.

"I can recall no instance in my time at the National Archives when a high-ranking official at an executive branch agency solely used a personal email account for the transaction of government business," Baron, who was at the agency from 2000 to 2013, told the newspaper.

A spokesman for Clinton, rumored to have her sights on the White House, said she complied with the "letter and spirit of the rules," but declined to say why a private email was used.

Spokesman Nick Merrill added that her emails were sent to other State Department employees with government email addresses. Clinton therefore had "every expectation they would be retained." He would not comment on emails sent to receivers in other agencies, the private sector or international leaders.  

Current Secretary of State John Kerry uses a government email account for all State Department communications.