Joint Base Andrews, which is home base for Air Force One, was briefly placed on lockdown Thursday after a report of an active shooter. There was no sign of an intruder after a thorough search.
No shots had been reported fired.
Officials said first responders had arrived at the Malcolm Grow Medical Facility and that personnel sheltered in place. At least three people wearing camouflage and helmets could be seen walking working dogs around the three-story building.
The base, which houses the presidential aircraft, is located in Prince George’s County and is about 20 miles outside of Washington.
The base had planned an active-shooter drill, but officials later told workers it was a “real-world” situation.
An employee at a medical facility at the base said he was told it was supposed to be drill that became real.
The Associated Press contributed to this report.