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

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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. People who saw the notification believed it to be true and alerted the police and NSA Bethesda, who assist with security on the military installation. Law enforcement, like the Montgomery County Police, treated the situation as a real active shooter scenario and closed down the hospital.

“While preparing for an upcoming drill, the notification system was inadvertently enacted without containing the words ‘EXERCISE’ or ‘DRILL,'” the U.S. Navy Twitter account said. “Individuals who saw the mass notification statement immediately notified NSA Bethesda security, where they responded accordingly and instituted an installation-wide active shooter response. On further investigation, they determined that the improper use of the system was the root cause and secured from the active-shooter response.”

Initially, the Navy’s Twitter account said that the alert was an “ad hoc drill,” before clarifying that it was an error. Specifics regarding who used the notification system and the circumstances around it were not provided.

Walter Reed is the largest military hospital in the U.S. and is listed as one of the first destinations in the continental United States to care for the wounded troops. It is not known exactly how many people were in the building during the active shooter alarm.

U.S. Rep. Dutch Ruppersberger of Maryland was inside Walter Reed during the alarm. He tweeted that he and about 40 others were in a hospital conference room when the shooting was reported. Ruppersberger posted multiple updates, saying the mood during the lock down was calm and that no one was given reason to believe the incident was a drill.

Mary Lock, a nurse at Walter Reed told the Associated Press that a loudspeaker repeated “Active shooter, this is not a drill,” as she was in lockdown for an hour.

Dr. Lindsey Migliore tweeted that security escorted everyone into rooms and made sure doors were closed. She also tweeted a video of what appears to be a SWAT team approach the building. She said that she was inside the hospital during the announcement and had not heard any shots fired.

On Twitter, as reports went out over the internet of an active shooter, family and friends of people who worked at the hospital were confused about what was actually happening.

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Write to Gina Martinez at gina.martinez@time.com