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.”

At 3:20 pm ET, the Facebook page was updated to say “the all clear has been given” and “no active shooter was found.”

At about the same time, the Department of Defense said Tuesday afternoon that there was no active shooter at Walter Reed and that reports of a shooter at the center were part of an exercise.

Pentagon spokesperson Lt. Col. Audricia Harris told CNN just after 3:15 p.m. ET Tuesday that the situation at Walter Reed was an exercise.

The US Navy said via Twitter at around the same time that it had confirmed there was no active shooter.

“Was an ad hoc drill by tenant command,” the Navy tweeted.

However, Naval Support Activity Bethesda contradicted that statement in a post on its verified Facebook page at about 4 p.m. ET.

“Around 2 p.m. today, Tuesday, Nov. 27, a call came into security at NSA Bethesda with the report of an active shooter situation in the basement of Bldg. 19 at Walter Reed Bethesda,” a Facebook post by the agency stated. “Security responded and cleared the building, finding no indication of an active shooter. After investigating the call and the origin, NSA Bethesda has determined that this was a false alarm and not part of a scheduled drill as has been reported.”

Walter Reed National Military Medical Center is in Bethesda, Maryland, just outside of Washington. According to its website, Walter Reed is the nation’s largest military medical center and has “roughly 7,000 dedicated staff members.”

Congressman on scene

Rep. Dutch Ruppersberger, D-Maryland, tweeted about the situation from inside the center as the incident unfolded.

“I am currently at Walter Reed Medical in Bethesda where we’ve been told there is an active shooter,” Ruppersberger tweeted. “I am currently safe in a conference room w/ approx 40 others.”

Later Tuesday, Ruppersberger said he remained confused over whether the incident had been a drill.

“If today’s active shooter investigation at Walter Reed Medical Center was a drill, I clearly did not get the memo and neither did the other people who were told to shelter in place with me, nor did Montgomery County Police and other agencies,” he tweeted.

“People were scared and upset. Drills are important and today was a valuable learning experience for me, but training exercises must be properly communicated. I will be following up to see what went wrong and how the process can be improved for the future. I’d like to thank law enforcement for their quick and professional response.”

Two individuals separately told CNN they were inside the building at Walter Reed just before 3 p.m. Tuesday and were told to continue to shelter in place. One individual said loudspeakers announced that it was still an active situation. The other individual reported hearing lots of sirens.