Navy: Active Shooter Alarm at Walter Reed Caused by ‘Improper Use of a Mass Notification System’

Reports of an active shooter at Maryland's Walter Reed National Military Medical Center on Tuesday afternoon stemmed from drill preparation gone wrong, according to the Navy.

People inside the hospital were unaware of this mistake, as Walter Reed went into lockdown for at least an hour.

The U.S. Navy confirmed on Twitter that the active shooter response was the result of the "improper use of a mass notification system" but did not specify where the alarm was set off or by whom.