Homeland Security Secretary Jeh Johnson called the incident an "unfolding situation."
The FBI was actively monitoring the situation.
A "barricade situation" occurred on the third floor of Building 1060, the Pentagon told Stars and Stripes.
"There were reports of an active shooter incident on base at Malcolm Grow medical facility at approximately 9 a.m. today, June 30, 2016," the base reported on its Facebook page. "Personnel on base have been directed to shelter in place.