A mass notification error led to reports of an active shooter Tuesday at Walter Reed National Military Medical Center outside Washington, the U.S. Navy said.

In a statement, the Navy said that the Naval Support Activity Bethesda notification system was inadvertently activated while preparing for an upcoming drill, without the words "exercise" or "drill." People who saw the notification contacted security at the Maryland base, which launched an active shooter response.

NSA Bethesda spokesman Jeremy Brooks said the incident "was an accident. It was not something that was planned."

NSA Bethesda tweeted at 2:15 p.m. that an active shooter had been reported in the basement of a building. The all clear came about an hour later, after security found "no indication" of an active shooter, according to NSA Bethesda.

After the all clear, the U.S. Navy tweeted that an "ad hoc drill" had been conducted at the base, but Brooks said there was no drill.

"It was a genuine false alarm and not intended to cause any harm," Brooks said.

U.S. Rep. Dutch Ruppersberger of Maryland, who had tweeted that he and about 40 other people were in a hospital conference room when the shooter was reported, said he'd be following up on the cause of the confusion.

"People were scared and upset. Drills are important and today was a valuable learning experience for me, but training exercises must be properly communicated," he wrote.

A nurse at Walter Reed, Mary Lock, said she and other employees remained locked down in a second-floor clinic for an hour after hearing this repeated announcement over a loudspeaker: "Active shooter, this is not a drill!"

Lock, 58, said they have had drills for events like this, so she didn't panic.

"It is nerve-wracking as all get out," she said with a laugh as she left work to catch a bus.

The Montgomery County Police Department also responded to the call, sending units after a request for assistance in what seemed like a legitimate report of an active shooter, spokeswoman Lucille Baur said.

"There was no indication from the call that this could be a training exercise," she said.

Walter Reed is the nation's largest military hospital and, according to its website, is among the first stops in the continental United States for troops wounded in combat.

NSAB oversees operational support for its major tenants at the base, including Walter Reed.