(CNN) An "improper" mass alert sparked a major scare over an active shooter at Walter Reed National Military Medical Center, the Navy said Tuesday evening.

The Navy's statement attributed confusion earlier in the day at the key military medical facility outside Washington to a mistakenly sent notification. The statement came after messages from the Navy and the Pentagon called the incident the result of a drill, while a local military agency called it an active shooter incident.

"While preparing for an upcoming drill, the notification system was inadvertently enacted without containing the words 'EXERCISE' or 'DRILL,'" the Navy said just before 6 p.m. ET. "Individuals who saw the mass notification statement immediately notified NSA Bethesda security, where they responded accordingly and instituted an installation-wide active shooter response."

The confusion started Tuesday afternoon when the Montgomery County Police Department's press office said it had received a call to assist on a report of an active shooter at Walter Reed.

Shortly after 2 p.m., the Facebook account for Naval Support Activity Bethesda noted "there is a report of an active shooter" in the basement of a building at Walter Reed. As the hour continued, the account said base security had cleared the basement and that there was "no indication so far of an active shooter."

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