At least one person has reportedly been shot and wounded and one suspect taken into custody at the US National Security Agency’s headquarters.
Gunfire rang out after a black SUV pulled up at the entrance to the facility in Fort Meade, Maryland.
An NSA spokesman told local media the area outside America’s electronic espionage agency was now secure after a “security incident”.
Earlier reports said that as many as three people had been wounded.
But Fort Meade police spokesman Larry Whitley said one person had been injured and transported to hospital.
It is unclear whether shots were fired by a suspect or by law enforcement.
But CBS News images showed the black sports utility vehicle with what looked like bullet holes in the front windscreen.
It appeared to have crashed into some NSA-stamped concrete barricade blocks outside the US Army installation.
NBC News said its helicopter could see police surrounding a handcuffed man sitting on the ground.
The FBI’s Baltimore office said on Twitter that it had sent agents to investigate the incident.
Local emergency services said on Twitter they also responded to the scene.
The agency said on Twitter: “NSA police and local law enforcement are addressing an incident that took place this morning at one of NSA’s secure vehicle entry gates.
“The situation is under control and there is no ongoing security of safety threat.”
Another statement from the NSA public affairs office said “a security incident” happened at one of its checkpoints at around 07:00 local time (12:00 GMT)
US President Donald Trump has been briefed on the incident.
“Our thoughts and prayers are with everyone that has been affected,” said a White House spokeswoman, Lindsay Walters.
In March 2015, one person was killed and another seriously wounded after driving a vehicle up to the NSA’s gate, ignoring commands to stop.
News reports later suggested the two occupants of the vehicle may have been under the influence of drugs after attending a party.
Despite highway signage, it is not unusual for motorists to take a wrong turn and end up at one of the heavily guarded gates to the sprawling NSA complex, where armed federal officers direct them to turn around.