House Committee on Oversight and Government Reform Committee Chairman Rep. Jason Chaffetz is threatening to issue a subpoena if the Department of Defense (DOD) doesn’t divulge how Secretary of Defense Ashton Carter used his personal email account on the job.

The Utah Republican first requested information on Carter’s emails Dec. 18, 2015, and set Jan. 8, 2016 as the response deadline. Defense officials have yet to provide any of the requested materials to the committee, according to a letter Chaffetz sent to Carter Friday. Chaffetz again demanded to know whether any emails Carter sent or received on his personal account were classified.

“Now, three months after my initial request, it is difficult to understand why the department has not been willing to provide detailed answers, and the department has not asserted a valid reason to withhold that information from the committee,” Chaffetz wrote, giving the DOD a March 25, 2016 deadline.

“Otherwise, the committee may use [a subpoena] to obtain documents and communications that will answer the questions that were initially raised on December 18, 2015,” he said.

The New York Times first reported in December that Carter relied on a personal email account to conduct government business during his first few months at the Pentagon. He continued using that email for at least two months after The Times reported presidential candidate Hillary Clinton sent and received emails on her personal account as secretary of state.

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