WASHINGTON (NBC) - Montgomery County Police are at Walter Reed Medical Center, where they are investigating the report of a single shot fired on campus. 

Police say they have not confirmed that a shot was fired.

"This has been going on for almost two hours now. We’ve been in hiding. We had to basically evacuate all the hallways, turn off all the lights," said a worker who identified himself only as Sam. "People are getting a little anxious."

The call came in at 10:48 a.m. Monday, police said. Employees were told to shelter in place via a text alert.

"We haven't confirmed that it's a gunshot, but just that it's the sound," said Capt. Paul Starks of Montgomery County Police.

Officers set up a perimeter and are working to determine whether there was a shot fired and if someone is there "with intent to commit violence," Starks said.

A 911 caller reported hearing one apparent shot inside Building 1, the original hospital on the campus in Bethesda, Montgomery County Police spokeswoman Natasha Plotnikov said, according to the Associated Press.

U.S. Navy Department of Defense and National Institutes of Health security personnel have also responded to the scene, the U.S. Navy tweeted.

Wisconsin Avenue is closed in both directions at Cedar Lane as the investigation continues.

On Thursday morning, employees were told to shelter in place at the Washington Navy Yard, where the initial report of a shooting triggered vivid memories of a 2013 deadly mass shooting at the naval installation.

D.C. police said a woman called from inside a Navy Yard building to report that she might have heard sounds of gunshots that morning. Employees were told to shelter in place as investigators swept the buildings and ultimately found no sign of a shooting.

"We don't believe that it was a malicious hoax or incident like that," D.C. Police Chief Cathy Lanier said of the Navy Yard incident, adding that the employee was doing what she was supposed to do.