BETHESDA, Md. (AP) — A report of an active shooter Tuesday at Walter Reed National Military Medical Center outside Washington was a "genuine false alarm," a spokesman said.

Naval Support Activity Bethesda spokesman Jeremy Brooks said officials at the Maryland base were looking into what caused the report.

NSAB 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 NSAB.

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. "We're still trying to determine how the call came to be."

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, later tweeted that "at no point was there any indication that this was a drill."

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.