Story highlights The FBI is looking into emails regarding the CIA's drone program as it investigated Hillary Clinton's use of a private email server

The drone program, while widely discussed in public and in the media, is still considered classified

Washington (CNN) A series of emails about the CIA's drone program are among those being investigated by the FBI as part of its probe into Hillary Clinton's use of a private email server while secretary of state, a law enforcement official tells CNN.

Those emails, sent in late 2011 and early 2012, involve a discussion by the then-U.S. ambassador to Pakistan about a planned U.S.-led drone strike in that country, and took place largely on what's called "low side," or non-classified, State Department email system. The planned drone strike was later canceled.

The Wall Street Journal was first to report the details of planned drone strike email discussion.

The exchange in question took place over the December holidays when multiple officials were away from the office and without access to their classified email, and some of the emails were ultimately sent to Secretary Clinton's private email server, according to the official.

The FBI has not yet interviewed Clinton as part of its investigation. As CNN first reported, investigators have not found evidence to support criminal charges against Clinton and none are expected, but no final determination will be made until that interview has taken place.

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