Defense Secretary Ash Carter used his personal email account to conduct some of his professional correspondence during his first months on the job earlier this year, the Pentagon admitted late Wednesday.

Carter's use of the personal account was first reported by The New York Times, which said that he had been confronted about his email habits by White House Chief of Staff Denis McDonough this past May, three months after Carter took office as defense secretary.

Pentagon spokesman Peter Cook released a statement saying that Carter believes his use of personal email for work-related business was a mistake. Cook declined to say whether it was a violation of Pentagon email policies. Cook also said Carter stopped the practice, but Cook did not say when.

Carter also acknowledged the move was a mistake in an interview with CBS' “This Morning” on Thursday. He said he occasionally used his iPhone to send messages to immediate staff, but stressed no classified information was involved.

The Times reported that Carter was assigned a government email account when he assumed his office in February, but continued to conduct most of his business on his private account, often sending messages via his iPhone or iPad. According to the paper, a former aide to Carter said that his boss used his personal account so often during that period that staffers feared he would be hacked.

Pentagon policy since 2012 has been to bar all employees from conducting government business on personal email. Last year, a law signed by President Obama barred federal officials from receiving or sending emails from personal accounts unless the messages were either copied or forwarded into government accounts within 20 days. It was not immediately clear whether Carter followed that directive.

The Times report comes in the midst of an FBI investigation into whether Hillary Clinton mishandled classified information by using a private account for all her emails while secretary of state. According to the Times, Carter continued to use his personal email at least two months after Clinton's practices were revealed in March. 

The Times said the emails it received under the Freedom of Information Act were exchanges between Carter and Eric Fanning, who was his chief of staff at the time and is now the acting secretary of the Army.

The emails were on a variety of work-related topics, the Times said, including speeches, meetings and news media appearances. In one such email, Carter discussed how he had mistakenly placed a note card in a "burn bag," the Times reported. Such bags are typically used to destroy classified documents.

Cook said Carter "does not use his personal email or official email for classified material. The Secretary has a secure communications team that handles his classified information and provides it to him as necessary."

Carter "takes his responsibilities with regard to classified material very seriously," Cook said.

The Associated Press contributed to this report.

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