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Flynn, 57, served as an advisor to Trump during his presidential campaign and was highly critical of Democratic presidential candidate Hillary Clinton and her use of a private email server while secretary of state.
He is a retired U.S. Army three-star general, was assistant director of national intelligence under President Barack Obama, but was fired in 2014. In Flynn's book, "The Field of Fight," Flynn claimed he was fired for speaking out against policies of the Obama administration, but others claim Flynn was pushed out because of his combative management style.
The Associated Press contributed to this report.