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.
CBS News images showed the black sports utility vehicle with what appeared to be bullet holes in the front windscreen.
It appeared to have crashed into the NSA-stamped concrete barricade blocks outside the base.
NBC News reported that its helicopter could see police surrounding a handcuffed man who was 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.
ABC News quoted an NSA spokesman as saying: “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 or safety threat.”
Another statement from the NSA public affairs office said: “At approximately 7am this morning, there was a security incident at one of the National Security Agency checkpoints.
“The NSA Police force is currently investigating.”
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.