Police opened fire and one suspect died Monday after a car tried to ram its way into the headquarters of US National Security Agency, America's electronic eavesdropper.
One police officer and a second suspect were hurt in the incident, the NSA's director for strategic communications Jonathan Freed said.
It was not immediately clear whether the suspects were shot or wounded when their car crashed into a police security vehicle.
An FBI spokeswoman said the incident at Fort Meade, the spy agency's super-secure base in suburban Maryland outside Washington, was not believed to be "related to terrorism."
According to Freed, a vehicle carrying two individuals was directed to turn back after it made an unauthorized attempt to enter the base.
Instead, the car accelerated toward an NSA vehicle blocking the road. Police opened fire but the car crashed into the security truck.
"One of the unauthorized vehicle's occupants died on the scene. The cause of death has not been determined," Freed said, in a statement.
"One NSA police officer was injured and taken to a local hospital. The incident was contained to the vehicle control point."
The FBI has taken charge of the investigation and the White House said President Barack Obama had been briefed on the situation.
News footage from helicopters showed a crash-damaged police vehicle and a civilian vehicle outside a main gate, and an injured person being transferred to an ambulance.
The FBI said it has opened an investigation, joining other law enforcement agencies and deploying agents to the scene to gather evidence and interview witnesses.
"The shooting scene is contained and we do not believe it is related to terrorism," said Amy Thoreson, a spokeswoman for the FBI's Baltimore office.
"We are working with the US attorney's office in Maryland to determine if federal charges are warranted," she said.
About 11,000 military personnel and 29,000 civilians work at Fort Meade, which also houses the headquarters of the US Cyber Command and other military units.
Fort Meade said all personnel and residents on the base were safe.
The NSA specializes in codebreaking and electronic surveillance, operating a global network of satellite surveillance, land listening stations and online data collection.
It has been the focus of intense controversy since early 2013, when former contractor Edward Snowden revealed the huge scope of its eavesdropping in a leak to the media.
Washington has denounced Snowden's document dump and subsequent flight to Russia, but Obama's administration has agreed to reform some of rules governing data interception.
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