Police were investigating a report of a gunshot heard on Monday at Walter Reed National Military Medical Center in suburban Washington.
The U.S. Navy said employees at the sprawling hospital's campus in Bethesda, Maryland, were told to shelter in place as police and security personnel responded to an unconfirmed report of an active shooter.
The Walter Reed report came after an unfounded report of a gunshot at the Washington Navy Yard sparked a massive police response on Thursday before the Independence Day holiday.
The Montgomery County Police Department said it had received a telephone call at 10:48 a.m. EDT with a report of a gunshot at Walter Reed. Police have not confirmed that report.
Helicopter camera footage from the local NBC TV affiliate showed officers in body armor and helmets deploying across the hospital grounds.