A tenant command inadvertently set off a mass notification system on Tuesday at Walter Reed National Military Medical Center, causing widespread confusion and early conflicting reports about a possible active shooter on campus.

The incident led to a lockdown on the sprawling Bethesda, Md.,-based campus, where people were urged to hide in safe areas as authorities investigated the scene. Initial reports said the alarm was part of a drill, but by nightfall officials said the alarm had been a mistake.