Last Updated Nov 27, 2018 5:28 PM EST
An active shooter scare Tuesday at Walter Reed National Military Medical Center in Bethesda, Maryland, turned out to be a drill gone awry, Pentagon officials said. The mixup prompted a large police response, leading workers and patients to take shelter.
The Navy said a unit at Walter Reed conducted an “ad hoc drill” and there was never an active shooter. An official said the unit accidentally triggered the alert and failed to notify the proper authorities. Officials then reacted as if the situation was the real thing.
The Naval Support Activity Bethesda (NSAB) said security received a call at 2 p.m. about an active shooter in the basement of Building 19. At 2:15 p.m. ET, the NSAB posted to Facebook, urging people in the area to stay away or find the nearest structure, building or vehicle that “provides a measure of protection.”
Security personnel responded but found no indication of a shooter. However, after giving the all-clear Tuesday afternoon, the NSAB denied the incident was part of a scheduled drill.
“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,” NSAB tweeted.
But Pentagon officials said a subordinate command was planning a drill when someone accidentally triggered an alert, prompting the response.

People leave Walter Reed National Military Medical Center in Washington, D.C., on November 27, 2018.
BRENDAN SMIALOWSKI/AFP/Getty
Rep. Dutch Ruppersberger, D-Md., tweeted he was at the scene shortly after 2:30 p.m. “We’ve been told there is an active shooter,” he wrote. “I am currently safe in a conference room with approximately 40 others.”
He later tweeted: “We’ve been given the all clear at Walter Reed — at no point was there any indication that this was a drill.” The NSAB said security had cleared the basement and there was “no indication so far of an active shooter.”
© 2018 CBS Interactive Inc. All Rights Reserved.