Several people have been injured and a suspect was taken into custody after a car crashed outside the US National Security Agency's headquarters.
Gunfire rang out after the black SUV approached the facility in Fort Meade, Maryland, without authorisation.
There is no indication yet of any link to terrorism, officials said.
An NSA spokesman said it was unclear if the shots had been fired by law enforcement officers or the suspect, adding that the scene was now secure.
NSA officials said the injuries did not appear to be related to the gunfire.
They added that several people were taken to hospital from the US Army base, which is about 30 miles (48km) north-east of Washington DC, after Wednesday morning's incident.
CBS News images showed the black sports utility vehicle with what looked like bullet holes in the front windscreen and its air bags appeared deployed.
It appeared to have collided into some NSA-stamped concrete barricade blocks outside the installation.
NBC News said its helicopter could see police surrounding a handcuffed man sitting on the ground.