Washington (CNN) Chief Pentagon spokeswoman Dana White is leaving her post, a move that coincides with Secretary of Defense James Mattis stepping down but also comes amid an internal Defense Department investigation into her conduct.

"I appreciate the opportunity afforded to me by this administration to serve alongside Secretary Mattis, our Service members and all the civilians who support them," White tweeted Monday. "It has been my honor and privilege. Stay safe and God bless."

White also sent a farewell message to Defense Department public affairs personnel Monday.

"It is with a heavy heart that I have submitted my letter of resignation to Secretary Mattis. I am grateful to the administration for giving me the opportunity to serve alongside Secretary Mattis, the brave men and women in uniform, and all of the civilians who support them," the message read, according to a copy obtained by CNN.

On Tuesday, Charles Summers Jr., a principal deputy assistant to the secretary of defense in White's office, will assume the role of acting assistant to the secretary of defense for public affairs, according to the Pentagon.

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