He had no timeline for when clearance activities would be completed.
Congressman Dutch Ruppersberger of Maryland said in the tweet that he and about 40 other people were in a conference room at the hospital.
The Montgomery County Police Department was called to the scene, Officer Rick Goodale said.
"We have officers out there, but as of yet we haven't been able to confirm this report," he said.
Walter Reed is America's largest military hospital and according to its website, is among the first stops in the continental United States for troops wounded in combat.
NSAB oversees operational support for its major tenants at the base, including Walter Reed.
-AP